diff --git a/changelog.d/ia-2018-misc-deduction.fixed.md b/changelog.d/ia-2018-misc-deduction.fixed.md
new file mode 100644
index 00000000000..13ce8ba8d49
--- /dev/null
+++ b/changelog.d/ia-2018-misc-deduction.fixed.md
@@ -0,0 +1 @@
+Keep Iowa's 2018 miscellaneous itemized deduction above 2% of federal AGI, which Iowa did not suspend until 2019, and apply Iowa's 2018 limitation on the itemized deductions of filers with federal AGI above the IA 104 applicable amounts.
diff --git a/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/agi_rate.yaml b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/agi_rate.yaml
new file mode 100644
index 00000000000..47f9f3a975b
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/agi_rate.yaml
@@ -0,0 +1,12 @@
+description: Iowa reduces itemized deductions by this fraction of federal adjusted gross income above the applicable amount, up to the itemized deductions cap.
+values:
+  2018-01-01: 0.03
+metadata:
+  unit: /1
+  period: year
+  label: Iowa itemized deductions limitation adjusted gross income rate
+  reference:
+    - title: 2018 IA 104 Iowa Itemized Deductions Worksheet, line 10
+      href: https://revenue.iowa.gov/media/2335/download?inline=#page=1
+    - title: 26 U.S. Code § 68(a)(1) (2014 edition), in effect on January 1, 2015
+      href: https://www.govinfo.gov/content/pkg/USCODE-2014-title26/html/USCODE-2014-title26-subtitleA-chap1-subchapB-partI-sec68.htm
diff --git a/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/applicable_amount.yaml b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/applicable_amount.yaml
new file mode 100644
index 00000000000..e5e9ab9b803
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/applicable_amount.yaml
@@ -0,0 +1,24 @@
+description: Iowa limits the itemized deductions of filers whose federal adjusted gross income exceeds this amount, based on federal filing status.
+metadata:
+  propagate_metadata_to_children: true
+  label: Iowa itemized deductions limitation applicable amount
+  period: year
+  unit: currency-USD
+  breakdown:
+    - filing_status
+  reference:
+    - title: 2018 IA 104 Iowa Itemized Deductions Worksheet, line 8
+      href: https://revenue.iowa.gov/media/2335/download?inline=#page=1
+    - title: Rev. Proc. 2017-58, section 3.15, applicable amounts under section 68(b) for 2018
+      href: https://www.irs.gov/pub/irs-drop/rp-17-58.pdf#page=14
+
+SINGLE:
+  2018-01-01: 266_700
+JOINT:
+  2018-01-01: 320_000
+SEPARATE:
+  2018-01-01: 160_000
+HEAD_OF_HOUSEHOLD:
+  2018-01-01: 293_350
+SURVIVING_SPOUSE:
+  2018-01-01: 320_000
diff --git a/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/itemized_deduction_rate.yaml b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/itemized_deduction_rate.yaml
new file mode 100644
index 00000000000..288542624dd
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/itemized_deduction_rate.yaml
@@ -0,0 +1,12 @@
+description: Iowa caps the reduction of itemized deductions at this fraction of the itemized deductions subject to the limitation.
+values:
+  2018-01-01: 0.8
+metadata:
+  unit: /1
+  period: year
+  label: Iowa itemized deductions limitation cap rate
+  reference:
+    - title: 2018 IA 104 Iowa Itemized Deductions Worksheet, line 6
+      href: https://revenue.iowa.gov/media/2335/download?inline=#page=1
+    - title: 26 U.S. Code § 68(a)(2) (2014 edition), in effect on January 1, 2015
+      href: https://www.govinfo.gov/content/pkg/USCODE-2014-title26/html/USCODE-2014-title26-subtitleA-chap1-subchapB-partI-sec68.htm
diff --git a/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/retained.yaml b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/retained.yaml
new file mode 100644
index 00000000000..438b363bc0a
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/retained.yaml
@@ -0,0 +1,20 @@
+description: Iowa keeps the limitation on the itemized deductions of high-income filers, which federal law suspends from 2018, if this is true.
+values:
+  2017-01-01: false
+  2018-01-01: true
+  2019-01-01: false
+metadata:
+  unit: bool
+  period: year
+  label: Iowa retains the itemized deductions limitation
+  reference:
+    - title: Iowa Code 422.3(5) (2018), Internal Revenue Code as amended and in effect on January 1, 2015
+      href: https://www.legis.iowa.gov/docs/code/2018/422.3.pdf#page=1
+    - title: 2018 Iowa Acts, chapter 1161, sections 58, 63, 66 and 69
+      href: https://www.legis.iowa.gov/docs/acts/2018/CH1161.pdf#page=14
+    - title: 2018 IA 1040 expanded instructions, Schedule A line 26
+      href: https://revenue.iowa.gov/media/2228/download?inline=#page=55
+    - title: 2018 IA 104 Iowa Itemized Deductions Worksheet
+      href: https://revenue.iowa.gov/media/2335/download?inline=#page=1
+    - title: 2019 IA 1040 Schedule A, line 21
+      href: https://revenue.iowa.gov/media/2490/download?inline=#page=1
diff --git a/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/misc_deduction_retained.yaml b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/misc_deduction_retained.yaml
new file mode 100644
index 00000000000..e562cacd79f
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/misc_deduction_retained.yaml
@@ -0,0 +1,20 @@
+description: Iowa keeps the miscellaneous itemized deduction above 2% of adjusted gross income, which federal law suspends from 2018, if this is true.
+values:
+  2017-01-01: false
+  2018-01-01: true
+  2019-01-01: false
+metadata:
+  unit: bool
+  period: year
+  label: Iowa retains the miscellaneous itemized deduction
+  reference:
+    - title: Iowa Code 422.3(5) (2018), Internal Revenue Code as amended and in effect on January 1, 2015
+      href: https://www.legis.iowa.gov/docs/code/2018/422.3.pdf#page=1
+    - title: Iowa Code 422.9(2) (2018), miscellaneous expenses deductible under the Internal Revenue Code
+      href: https://www.legis.iowa.gov/docs/code/2018/422.9.pdf#page=1
+    - title: 2018 IA 1040 expanded instructions, Schedule A job expenses and miscellaneous deductions
+      href: https://revenue.iowa.gov/media/2228/download?inline=#page=54
+    - title: 2018 IA Schedule A, lines 19 to 24
+      href: https://revenue.iowa.gov/media/2298/download?inline=#page=1
+    - title: 2019 IA 1040 expanded instructions, Schedule A
+      href: https://revenue.iowa.gov/media/2492/download?inline=#page=49
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_indiv.yaml b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_indiv.yaml
index d12afc7a38e..5bd746ec661 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_indiv.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_indiv.yaml
@@ -21,3 +21,38 @@
         state_code: IA
   output:
     ia_itemized_deductions_indiv: [24_000, 6_000]
+
+- name: 2018 spouses filing separately on a combined return split the limited total
+  period: 2018
+  input:
+    people:
+      person1:
+        is_tax_unit_head: true
+        ia_net_income: 300_000
+        real_estate_taxes: 8_000
+        deductible_mortgage_interest: 14_000
+        ia_withheld_income_tax: 15_000
+      person2:
+        is_tax_unit_spouse: true
+        ia_net_income: 100_000
+        ia_withheld_income_tax: 5_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        filing_status: JOINT
+        adjusted_gross_income: 400_000
+        charitable_deduction: 6_000
+        medical_expense_deduction: 4_000
+        state_sales_tax: 0
+        local_sales_tax: 0
+    households:
+      household:
+        members: [person1, person2]
+        state_code: IA
+  output:
+    # 2018 IA 104 line 21 with the $320,000 joint amount: 30,600 (see
+    # ia_itemized_deductions_reduction.yaml). 2018 IA Schedule A lines 29 to
+    # 33 split it by net income: 300,000 / 400,000 = 75.0%, so 22,950 and
+    # 7,650: https://revenue.iowa.gov/media/2298/download?inline=#page=1
+    ia_itemized_deductions_unit: 30_600
+    ia_itemized_deductions_indiv: [22_950, 7_650]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_reduction.yaml b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_reduction.yaml
new file mode 100644
index 00000000000..efaa2011ac1
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_reduction.yaml
@@ -0,0 +1,281 @@
+# 2018 IA 104 Iowa Itemized Deductions Worksheet:
+# https://revenue.iowa.gov/media/2335/download?inline=#page=1
+# 2018 IA Schedule A line 26 sends filers with federal AGI over $160,000 to
+# the worksheet: https://revenue.iowa.gov/media/2298/download?inline=#page=1
+# Line 1 is Schedule A lines 1 to 25, line 2 the Iowa income tax on federal
+# Schedule A line 5a, line 4 medical, investment interest and casualty losses.
+
+- name: 2018 single filer at $150,000 is not limited
+  period: 2018
+  input:
+    filing_status: SINGLE
+    adjusted_gross_income: 150_000
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    charitable_deduction: 3_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # Schedule A line 26: federal AGI is not over $160,000, so add the lines.
+    ia_itemized_deductions_pre_limitation: 18_000
+    ia_itemized_deductions_reduction: 0
+    ia_itemized_deductions_unit: 18_000
+
+- name: 2018 separate filer at exactly $160,000 is not limited
+  period: 2018
+  input:
+    filing_status: SEPARATE
+    adjusted_gross_income: 160_000
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    charitable_deduction: 3_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # IA 104 line 9: line 8 ($160,000) is not less than line 7 ($160,000).
+    ia_itemized_deductions_reduction: 0
+    ia_itemized_deductions_unit: 18_000
+
+- name: 2018 separate filer above $160,000 is limited
+  period: 2018
+  input:
+    filing_status: SEPARATE
+    adjusted_gross_income: 170_000
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    charitable_deduction: 3_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # Lines 1, 3 and 5: 18,000; line 6: 0.8 * 18,000 = 14,400.
+    # Line 9: 170,000 - 160,000 = 10,000; line 10: 3% = 300; line 11: 300.
+    # Line 12: 18,000 - 300 = 17,700; line 2 is zero, so line 21 = 17,700.
+    ia_itemized_deductions_reduction: 300
+    ia_itemized_deductions_unit: 17_700
+
+- name: 2018 single filer above $160,000 but below $266,700 is not limited
+  period: 2018
+  input:
+    filing_status: SINGLE
+    adjusted_gross_income: 170_000
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    charitable_deduction: 3_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # IA 104 line 9: line 8 ($266,700 for single) is not less than line 7.
+    ia_itemized_deductions_reduction: 0
+    ia_itemized_deductions_unit: 18_000
+
+- name: 2018 head of household above $293,350 is limited
+  period: 2018
+  input:
+    filing_status: HEAD_OF_HOUSEHOLD
+    adjusted_gross_income: 303_350
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    charitable_deduction: 3_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # Line 9: 303,350 - 293,350 = 10,000; line 10: 300; line 6: 14,400.
+    # Line 11: 300; line 21: 18,000 - 300 = 17,700.
+    ia_itemized_deductions_reduction: 300
+    ia_itemized_deductions_unit: 17_700
+
+- name: 2018 surviving spouse above $320,000 is limited
+  period: 2018
+  input:
+    filing_status: SURVIVING_SPOUSE
+    adjusted_gross_income: 330_000
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    charitable_deduction: 3_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # Line 9: 330,000 - 320,000 = 10,000; line 10: 300; line 6: 14,400.
+    # Line 11: 300; line 21: 18,000 - 300 = 17,700.
+    ia_itemized_deductions_reduction: 300
+    ia_itemized_deductions_unit: 17_700
+
+- name: 2018 joint filer shares the reduction with Iowa income tax
+  period: 2018
+  absolute_error_margin: 0.01
+  input:
+    filing_status: JOINT
+    adjusted_gross_income: 400_000
+    real_estate_taxes: 8_000
+    deductible_mortgage_interest: 14_000
+    charitable_deduction: 6_000
+    medical_expense_deduction: 4_000
+    ia_withheld_income_tax: 20_000
+    state_sales_tax: 0
+    local_sales_tax: 0
+    state_code: IA
+  output:
+    # Line 1: 8,000 + 14,000 + 6,000 + 4,000 = 32,000; line 2: 20,000;
+    # line 3: 52,000; line 4: 4,000 (medical); line 5: 48,000;
+    # line 6: 38,400; line 9: 400,000 - 320,000 = 80,000; line 10: 2,400;
+    # line 11: 2,400; line 12: 49,600; line 15: 49,600 - 4,000 = 45,600;
+    # line 17: 52,000 - 4,000 = 48,000; line 18: 95.0%;
+    # line 20: 20,000 * 95% = 19,000; line 21: 49,600 - 19,000 = 30,600.
+    ia_itemized_deductions_pre_limitation: 32_000
+    ia_itemized_deductions_reduction: 1_400
+    ia_itemized_deductions_unit: 30_600
+
+- name: 2018 line 18 rounds the Iowa income tax share to a tenth of a percent
+  period: 2018
+  absolute_error_margin: 0.01
+  input:
+    filing_status: JOINT
+    adjusted_gross_income: 400_000
+    real_estate_taxes: 8_000
+    deductible_mortgage_interest: 14_000
+    charitable_deduction: 4_000
+    medical_expense_deduction: 4_000
+    ia_withheld_income_tax: 20_000
+    state_sales_tax: 0
+    local_sales_tax: 0
+    state_code: IA
+  output:
+    # Line 1: 30,000; line 2: 20,000; line 3: 50,000; line 4: 4,000;
+    # line 5: 46,000; line 6: 36,800; line 10: 2,400; line 11: 2,400;
+    # line 12: 47,600; line 15: 43,600; line 17: 46,000;
+    # line 18: 94.78% enters as 94.8%; line 20: 18,960;
+    # line 21: 47,600 - 18,960 = 28,640. The exact share gives 28,643.48.
+    ia_itemized_deductions_pre_limitation: 30_000
+    ia_itemized_deductions_reduction: 1_360
+    ia_itemized_deductions_unit: 28_640
+
+- name: 2018 rounding on line 18 can raise line 21 above line 1
+  period: 2018
+  absolute_error_margin: 0.01
+  input:
+    filing_status: JOINT
+    adjusted_gross_income: 500_000
+    medical_expense_deduction: 12_000
+    investment_interest_expense: 8_000
+    ia_withheld_income_tax: 9_700
+    state_sales_tax: 0
+    local_sales_tax: 0
+    state_code: IA
+  output:
+    # Line 1: 20,000; line 3: 29,700; line 4: 20,000; line 5: 9,700;
+    # line 6: 7,760; line 10: 5,400; line 11: 5,400; line 12: 24,300;
+    # line 15: 4,300; line 17: 9,700; line 18: 44.33% enters as 44.3%;
+    # line 20: 4,297.10; line 21: 24,300 - 4,297.10 = 20,002.90.
+    ia_itemized_deductions_pre_limitation: 20_000
+    ia_itemized_deductions_reduction: -2.90
+    ia_itemized_deductions_unit: 20_002.90
+
+- name: 2018 line 18 rounds an exact half up
+  period: 2018
+  absolute_error_margin: 0.01
+  input:
+    filing_status: JOINT
+    adjusted_gross_income: 410_000
+    real_estate_taxes: 10_000
+    deductible_mortgage_interest: 10_000
+    ia_withheld_income_tax: 20_000
+    state_sales_tax: 0
+    local_sales_tax: 0
+    state_code: IA
+  output:
+    # Line 1: 20,000; line 2: 20,000; line 3: 40,000; line 4: 0; line 5: 40,000;
+    # line 6: 32,000; line 9: 90,000; line 10: 2,700; line 11: 2,700;
+    # line 12: 37,300; line 15: 37,300; line 17: 40,000;
+    # line 18: 93.25% enters as 93.3%; line 20: 18,660;
+    # line 21: 37,300 - 18,660 = 18,640.
+    ia_itemized_deductions_reduction: 1_360
+    ia_itemized_deductions_unit: 18_640
+
+- name: 2018 joint filer with sales tax on federal Schedule A has no Iowa income tax on line 2
+  period: 2018
+  input:
+    filing_status: JOINT
+    adjusted_gross_income: 400_000
+    real_estate_taxes: 8_000
+    deductible_mortgage_interest: 14_000
+    charitable_deduction: 6_000
+    medical_expense_deduction: 4_000
+    ia_withheld_income_tax: 20_000
+    state_sales_tax: 25_000
+    local_sales_tax: 0
+    state_code: IA
+  output:
+    # Federal line 5a holds the larger sales tax, so line 2 is zero.
+    # Line 3: 32,000; line 5: 28,000; line 6: 22,400; line 11: 2,400;
+    # line 12 = line 21: 32,000 - 2,400 = 29,600.
+    ia_itemized_deductions_reduction: 2_400
+    ia_itemized_deductions_unit: 29_600
+
+- name: 2018 cap of 80% of the limited deductions binds
+  period: 2018
+  input:
+    filing_status: SINGLE
+    adjusted_gross_income: 1_000_000
+    real_estate_taxes: 4_000
+    deductible_mortgage_interest: 6_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # Line 5: 10,000; line 6: 8,000; line 9: 1,000,000 - 266,700 = 733,300;
+    # line 10: 21,999; line 11: 8,000; line 21: 10,000 - 8,000 = 2,000.
+    ia_itemized_deductions_reduction: 8_000
+    ia_itemized_deductions_unit: 2_000
+
+- name: 2018 Iowa income tax absorbs the reduction when only excluded deductions remain
+  period: 2018
+  absolute_error_margin: 0.01
+  input:
+    filing_status: JOINT
+    adjusted_gross_income: 500_000
+    medical_expense_deduction: 12_000
+    investment_interest_expense: 8_000
+    ia_withheld_income_tax: 10_000
+    state_sales_tax: 0
+    local_sales_tax: 0
+    state_code: IA
+  output:
+    # Line 1: 20,000; line 3: 30,000; line 4: 12,000 + 8,000 = 20,000;
+    # line 5: 10,000; line 6: 8,000; line 9: 180,000; line 10: 5,400;
+    # line 11: 5,400; line 12: 24,600; line 15: 4,600; line 17: 10,000;
+    # line 18: 46.0%; line 20: 4,600; line 21: 24,600 - 4,600 = 20,000.
+    ia_itemized_deductions_pre_limitation: 20_000
+    ia_itemized_deductions_reduction: 0
+    ia_itemized_deductions_unit: 20_000
+
+- name: 2018 limitation applies after the miscellaneous deduction
+  period: 2018
+  input:
+    filing_status: SEPARATE
+    adjusted_gross_income: 200_000
+    unreimbursed_business_employee_expenses: 10_000
+    real_estate_taxes: 6_000
+    deductible_mortgage_interest: 9_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # Schedule A line 24: 10,000 - 2% * 200,000 = 6,000; line 1: 21,000.
+    # Line 9: 40,000; line 10: 1,200; line 6: 16,800; line 11: 1,200.
+    # Line 21: 21,000 - 1,200 = 19,800.
+    ia_itemized_deductions_pre_limitation: 21_000
+    ia_itemized_deductions_reduction: 1_200
+    ia_itemized_deductions_unit: 19_800
+
+- name: 2019 Iowa follows the federal suspension of the limitation
+  period: 2019
+  input:
+    filing_status: SINGLE
+    adjusted_gross_income: 1_000_000
+    real_estate_taxes: 4_000
+    deductible_mortgage_interest: 6_000
+    ia_withheld_income_tax: 0
+    state_code: IA
+  output:
+    # 2019 IA Schedule A line 21 adds the lines with no worksheet.
+    ia_itemized_deductions_reduction: 0
+    ia_itemized_deductions_unit: 10_000
+
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_unit.yaml b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_unit.yaml
index 9494a5b10c4..ccf3b010c3c 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_unit.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_itemized_deductions_unit.yaml
@@ -95,3 +95,56 @@
     deductible_mortgage_insurance_premiums: 2_000
     interest_deduction: 12_000
     ia_itemized_deductions_unit: 12_000
+
+# 2018 IA Schedule A lines 19 to 24: unreimbursed employee expenses and tax
+# preparation fees above 2% of federal AGI, which the federal return suspends.
+- name: 2018 Iowa keeps the miscellaneous deduction above 2% of federal AGI
+  period: 2018
+  input:
+    unreimbursed_business_employee_expenses: 3_000
+    tax_preparation_fees: 500
+    adjusted_gross_income: 50_000
+    real_estate_taxes: 2_000
+    state_code: IA
+  output:
+    misc_deduction: 0
+    # 2,000 + (3,500 - 2% * 50,000)
+    ia_itemized_deductions_unit: 4_500
+
+- name: 2019 Iowa follows the federal suspension of the miscellaneous deduction
+  period: 2019
+  input:
+    unreimbursed_business_employee_expenses: 3_000
+    tax_preparation_fees: 500
+    adjusted_gross_income: 50_000
+    real_estate_taxes: 2_000
+    state_code: IA
+  output:
+    ia_itemized_deductions_unit: 2_000
+
+- name: 2018 Iowa miscellaneous deduction is zero when 2% of federal AGI exceeds the expenses
+  period: 2018
+  input:
+    filing_status: SINGLE
+    unreimbursed_business_employee_expenses: 3_000
+    tax_preparation_fees: 500
+    adjusted_gross_income: 200_000
+    real_estate_taxes: 2_000
+    state_code: IA
+  output:
+    # 2018 IA Schedule A lines 22 to 24: 3,500 - 2% * 200,000 = -500, so 0.
+    # Federal AGI is below the $266,700 single amount on the IA 104, line 8.
+    ia_itemized_deductions_unit: 2_000
+
+- name: 2017 Iowa takes the federal miscellaneous deduction once
+  period: 2017
+  input:
+    unreimbursed_business_employee_expenses: 3_000
+    tax_preparation_fees: 500
+    adjusted_gross_income: 50_000
+    real_estate_taxes: 2_000
+    state_code: IA
+  output:
+    # The federal deduction still applies in 2017: 3,500 - 2% * 50,000.
+    misc_deduction: 2_500
+    ia_itemized_deductions_unit: 4_500
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/test_ia_itemized_deductions_reduction_invariants.py b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/test_ia_itemized_deductions_reduction_invariants.py
new file mode 100644
index 00000000000..53a9372ea5d
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/test_ia_itemized_deductions_reduction_invariants.py
@@ -0,0 +1,219 @@
+"""Invariants of Iowa's 2018 limitation on itemized deductions.
+
+The 2018 IA 104 worksheet limits the 2018 IA Schedule A total of a filer whose
+federal adjusted gross income exceeds the applicable amount for the federal
+filing status (line 8):
+https://revenue.iowa.gov/media/2335/download?inline=#page=1
+Line 11 is the smaller of 80% of the deductions subject to the limitation
+(line 6) and 3% of the excess income (line 10). Lines 12 to 21 then remove
+the Iowa income tax on federal Schedule A line 5a in proportion, as
+701 IAC 41.11(1) does, at a share that line 18 enters to the nearest tenth of
+a percent:
+https://www.legis.iowa.gov/docs/iac/chapter/03-28-2018.701.41.pdf#page=10
+
+So each bound below allows ROUNDING of the Iowa income tax on line 2, the most
+that line 18's rounding moves line 21 from the exact share.
+
+Hypothesis draws deduction inputs, Iowa income tax, sales tax and federal AGI
+relative to the applicable amount, so every filing status gets limited cases.
+Each draw becomes one simulation with four tax units per case: the drawn
+case; the case without miscellaneous expenses ("fixed"), whose Schedule A
+total does not depend on AGI; "fixed" with higher AGI; and "fixed" with more
+Iowa income tax. For every tax unit:
+
+1. Accounting: ia_itemized_deductions_unit is the pre-limitation total minus
+   the reduction, because Schedule A line 26 takes IA 104 line 21.
+2. Never raises: apart from rounding, the reduction is never negative, so
+   the limitation never raises the deduction, because line 11 is the smaller
+   of two nonnegative amounts and the exact share is at most 100%.
+3. Bounded: apart from rounding, the reduction is at most 80% of the
+   deductions other than medical expenses and investment interest, because
+   line 6 caps line 11 at 80% of line 5 and Schedule A bears only its line-1
+   share of line 11.
+4. Threshold: no reduction when federal AGI is at most the applicable
+   amount, because line 9 then stops the worksheet.
+5. Monotone in AGI: with the deductions fixed, higher AGI never lowers the
+   reduction, because line 9 grows with line 7.
+6. Monotone in Iowa income tax: more Iowa income tax never raises the
+   reduction, because lines 15 to 20 give the Iowa income tax its share of
+   line 11.
+7. Worksheet: the reduction stays within rounding of line 11 times the
+   Schedule A share, (line 1 - line 4) / line 5, with line 1 and AGI taken
+   from the model and lines 2 to 11 restated here from the draw.
+
+Comparisons allow ten cents: float32 spacing is under seven cents below
+$1 million.
+"""
+
+import hypothesis
+import hypothesis.strategies as st
+import numpy as np
+import pytest
+
+from policyengine_us import Simulation
+
+YEAR = 2018
+# 2018 IA 104 line 8, by federal filing status.
+APPLICABLE_AMOUNT = {
+    "SINGLE": 266_700,
+    "JOINT": 320_000,
+    "SEPARATE": 160_000,
+    "HEAD_OF_HOUSEHOLD": 293_350,
+    "SURVIVING_SPOUSE": 320_000,
+}
+# 2018 IA 104 line 6.
+CAP_RATE = 0.8
+# 2018 IA 104 line 10.
+AGI_RATE = 0.03
+# 2018 IA 104 line 18 rounds to a tenth of a percent, half of which is 0.05%.
+ROUNDING = 0.0005
+TOLERANCE = 0.1
+VARIANTS = ("drawn", "fixed", "higher_agi", "more_iowa_tax")
+
+amount = st.integers(min_value=0, max_value=60_000)
+case_strategy = st.fixed_dictionaries(
+    {
+        "filing_status": st.sampled_from(sorted(APPLICABLE_AMOUNT)),
+        "agi_offset": st.integers(min_value=-50_000, max_value=1_000_000),
+        "agi_increase": st.integers(min_value=0, max_value=500_000),
+        "real_estate_taxes": amount,
+        "mortgage_interest": amount,
+        "charity": amount,
+        "medical": amount,
+        "investment_interest": amount,
+        "misc_expenses": amount,
+        "iowa_income_tax": st.integers(min_value=0, max_value=120_000),
+        "iowa_income_tax_increase": st.integers(min_value=0, max_value=60_000),
+        "sales_tax": amount,
+    }
+)
+
+
+def base_agi(case):
+    return max(0, APPLICABLE_AMOUNT[case["filing_status"]] + case["agi_offset"])
+
+
+def line_2(case, variant):
+    """Iowa income tax on federal Schedule A line 5a, as the draw sets it."""
+    iowa_income_tax = case["iowa_income_tax"]
+    if variant == "more_iowa_tax":
+        iowa_income_tax += case["iowa_income_tax_increase"]
+    return iowa_income_tax if iowa_income_tax >= case["sales_tax"] else 0
+
+
+def build_situation(cases):
+    people, tax_units, households = {}, {}, {}
+    for i, case in enumerate(cases):
+        for variant in VARIANTS:
+            key = f"{variant}_{i}"
+            person = f"person_{key}"
+            misc_expenses = case["misc_expenses"] if variant == "drawn" else 0
+            iowa_income_tax = case["iowa_income_tax"]
+            if variant == "more_iowa_tax":
+                iowa_income_tax += case["iowa_income_tax_increase"]
+            agi = base_agi(case)
+            if variant == "higher_agi":
+                agi += case["agi_increase"]
+            people[person] = {
+                "age": {YEAR: 45},
+                "real_estate_taxes": {YEAR: case["real_estate_taxes"]},
+                "deductible_mortgage_interest": {YEAR: case["mortgage_interest"]},
+                "investment_interest_expense": {YEAR: case["investment_interest"]},
+                "unreimbursed_business_employee_expenses": {YEAR: misc_expenses},
+                "ia_withheld_income_tax": {YEAR: iowa_income_tax},
+            }
+            tax_units[f"tax_unit_{key}"] = {
+                "members": [person],
+                "filing_status": {YEAR: case["filing_status"]},
+                "adjusted_gross_income": {YEAR: agi},
+                "charitable_deduction": {YEAR: case["charity"]},
+                "medical_expense_deduction": {YEAR: case["medical"]},
+                "state_sales_tax": {YEAR: case["sales_tax"]},
+                "local_sales_tax": {YEAR: 0},
+            }
+            households[f"household_{key}"] = {
+                "members": [person],
+                "state_code": {YEAR: "IA"},
+            }
+    return {"people": people, "tax_units": tax_units, "households": households}
+
+
+def calculate(cases):
+    simulation = Simulation(situation=build_situation(cases))
+    shape = (len(cases), len(VARIANTS))
+    return {
+        name: np.asarray(simulation.calculate(name, YEAR)).reshape(shape)
+        for name in (
+            "adjusted_gross_income",
+            "ia_itemized_deductions_pre_limitation",
+            "ia_itemized_deductions_reduction",
+            "ia_itemized_deductions_unit",
+        )
+    }
+
+
+@hypothesis.settings(
+    max_examples=20,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[hypothesis.HealthCheck.too_slow],
+)
+@hypothesis.given(st.lists(case_strategy, min_size=1, max_size=8))
+def test_ia_itemized_deductions_reduction_invariants(cases):
+    values = calculate(cases)
+    agi = values["adjusted_gross_income"]
+    pre = values["ia_itemized_deductions_pre_limitation"]
+    reduction = values["ia_itemized_deductions_reduction"]
+    unit = values["ia_itemized_deductions_unit"]
+    fixed, higher_agi, more_iowa_tax = (
+        VARIANTS.index(variant) for variant in ("fixed", "higher_agi", "more_iowa_tax")
+    )
+    for i, case in enumerate(cases):
+        excluded = case["medical"] + case["investment_interest"]
+        applicable_amount = APPLICABLE_AMOUNT[case["filing_status"]]
+        for j, variant in enumerate(VARIANTS):
+            context = (variant, case)
+            # Line 2 and the most that rounding moves the result.
+            iowa_tax = line_2(case, variant)
+            rounding = ROUNDING * iowa_tax
+            # 1. Accounting.
+            assert unit[i, j] == pytest.approx(
+                pre[i, j] - reduction[i, j], abs=TOLERANCE
+            ), context
+            # 2. Never raises, apart from rounding.
+            assert reduction[i, j] >= -rounding - TOLERANCE, context
+            assert unit[i, j] <= pre[i, j] + rounding + TOLERANCE, context
+            # 3. Bounded by line 6, apart from rounding.
+            assert (
+                reduction[i, j]
+                <= CAP_RATE * max(0, pre[i, j] - excluded) + rounding + TOLERANCE
+            ), context
+            # 4. Threshold.
+            if agi[i, j] <= applicable_amount:
+                assert reduction[i, j] == 0, context
+            # 7. Worksheet: lines 3, 5, 6, 9, 10 and 11, then the exact share.
+            line_5 = max(0, pre[i, j] + iowa_tax - excluded)
+            line_11 = min(
+                CAP_RATE * line_5, AGI_RATE * max(0, agi[i, j] - applicable_amount)
+            )
+            exact = line_11 * max(0, pre[i, j] - excluded) / line_5 if line_5 else 0
+            assert reduction[i, j] == pytest.approx(exact, abs=rounding + TOLERANCE), (
+                context
+            )
+        # The fixed variants share one Schedule A total.
+        assert pre[i, higher_agi] == pytest.approx(pre[i, fixed], abs=TOLERANCE), case
+        assert pre[i, more_iowa_tax] == pytest.approx(pre[i, fixed], abs=TOLERANCE), (
+            case
+        )
+        # 5. Monotone in AGI, apart from rounding at each of the two points.
+        fixed_rounding = ROUNDING * line_2(case, "fixed")
+        assert (
+            reduction[i, higher_agi]
+            >= reduction[i, fixed] - 2 * fixed_rounding - TOLERANCE
+        ), case
+        # 6. Monotone in Iowa income tax, apart from rounding at each point.
+        more_rounding = ROUNDING * line_2(case, "more_iowa_tax")
+        assert (
+            reduction[i, more_iowa_tax]
+            <= reduction[i, fixed] + fixed_rounding + more_rounding + TOLERANCE
+        ), case
diff --git a/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_pre_limitation.py b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_pre_limitation.py
new file mode 100644
index 00000000000..cdc1f9adb0c
--- /dev/null
+++ b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_pre_limitation.py
@@ -0,0 +1,53 @@
+from policyengine_us.model_api import *
+
+
+class ia_itemized_deductions_pre_limitation(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "Iowa itemized deductions for tax unit before the high-income limitation"
+    unit = USD
+    definition_period = YEAR
+    reference = (
+        "https://revenue.iowa.gov/sites/default/files/2022-01/IA1040%2841-001%29.pdf",
+        "https://revenue.iowa.gov/media/2650/download?inline",
+        "https://revenue.iowa.gov/sites/default/files/2023-01/2022IA1040%2841001%29.pdf",
+        "https://revenue.iowa.gov/media/2721/download?inline",
+        "https://revenue.iowa.gov/taxes/tax-guidance/individual-income-tax/iowa-nonconformity-taxpayer-certainty-and-disaster-tax-relief-act-2019",
+        # 2018 IA Schedule A, lines 1 to 25.
+        "https://revenue.iowa.gov/media/2298/download?inline=#page=1",
+    )
+    defined_for = StateCode.IA
+
+    def formula(tax_unit, period, parameters):
+        """
+        FROM THE 2021 INSTRUCTIONS (PAGE 46):
+          The $10,000 federal cap on the itemized deduction for state and
+        local taxes does not apply for Iowa purposes. Taxpayers may still
+        deduct eligible state and local taxes paid, independent of the
+        federal dollar limitation.
+        """
+        # compute tax unit's itemized deductions
+        p = parameters(period).gov.states.ia.tax.income.deductions.itemized
+        if p.applies_federal:
+            p_fed = parameters(period).gov.irs.deductions
+            return add(tax_unit, period, p_fed.itemized_deductions)
+        deductions = add(
+            tax_unit,
+            period,
+            ["itemized_deductions_less_salt", "real_estate_taxes"],
+        )
+        # Iowa did not adopt the retroactive extension of the mortgage
+        # insurance premium deduction for 2018 and 2019.
+        if p.disallowed_interest_components:
+            deductions = deductions - add(
+                tax_unit, period, p.disallowed_interest_components
+            )
+        if p.misc_deduction_retained:
+            # 2018: Iowa did not adopt the federal suspension of miscellaneous
+            # deductions above 2% of federal AGI (Schedule A lines 19 to 24),
+            # so they are added to the federal amounts, which leave them out.
+            p_misc = parameters(period).gov.irs.deductions.itemized.misc
+            expenses = tax_unit("total_misc_deductions", period)
+            floor = p_misc.floor * tax_unit("positive_agi", period)
+            deductions = deductions + max_(0, expenses - floor)
+        return deductions
diff --git a/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_reduction.py b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_reduction.py
new file mode 100644
index 00000000000..9acd938930f
--- /dev/null
+++ b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_reduction.py
@@ -0,0 +1,68 @@
+from policyengine_us.model_api import *
+
+
+class ia_itemized_deductions_reduction(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "Iowa itemized deductions reduction for high-income filers"
+    unit = USD
+    definition_period = YEAR
+    reference = (
+        # 2018 IA 104 Iowa Itemized Deductions Worksheet.
+        "https://revenue.iowa.gov/media/2335/download?inline=#page=1",
+        # 701 IAC 41.11 (IAC 2/28/18), PDF pages 9-10.
+        "https://www.legis.iowa.gov/docs/iac/chapter/03-28-2018.701.41.pdf#page=9",
+        # Iowa Code 422.9(2) (2018).
+        "https://www.legis.iowa.gov/docs/code/2018/422.9.pdf#page=1",
+    )
+    defined_for = StateCode.IA
+
+    def formula(tax_unit, period, parameters):
+        p = parameters(period).gov.states.ia.tax.income.deductions.itemized
+        if not p.limitation.retained:
+            return 0
+        # IA 104 line 1: Iowa Schedule A lines 1 to 25.
+        deductions = tax_unit("ia_itemized_deductions_pre_limitation", period)
+        # IA 104 line 2: Iowa income tax on federal Schedule A line 5a, which
+        # holds state income taxes only when they exceed sales taxes.
+        income_taxes = add(
+            tax_unit, period, ["state_withheld_income_tax", "local_income_tax"]
+        )
+        sales_taxes = add(tax_unit, period, ["state_sales_tax", "local_sales_tax"])
+        iowa_income_tax = where(
+            income_taxes >= sales_taxes,
+            add(tax_unit, period, ["ia_withheld_income_tax"]),
+            0,
+        )
+        # IA 104 line 4: medical expenses, investment interest, and casualty,
+        # theft and gambling losses, which section 68(c) leaves unlimited.
+        p_fed = parameters(period).gov.irs.deductions.itemized.limitation
+        excluded = add(tax_unit, period, p_fed.excluded_deductions)
+        # IA 104 lines 3 and 5. Lines 3 to 11 run in float64, so float32 error
+        # can't move line 18 across a half.
+        reducible = max_(0, deductions + iowa_income_tax - excluded).astype(np.float64)
+        # IA 104 lines 7 to 9, with the federal filing status.
+        agi = tax_unit("adjusted_gross_income", period)
+        filing_status = tax_unit("filing_status", period)
+        excess_agi = max_(
+            0, agi - p.limitation.applicable_amount[filing_status]
+        ).astype(np.float64)
+        # IA 104 lines 6, 10 and 11.
+        reduction = min_(
+            p.limitation.itemized_deduction_rate * reducible,
+            p.limitation.agi_rate * excess_agi,
+        )
+        # IA 104 lines 12 to 21 subtract the Iowa income tax in the share of
+        # line 5 that the reduction leaves. Line 18 enters that share to the
+        # nearest tenth of a percent; neither Iowa Code 422.9(2) nor 701 IAC
+        # 41.11(1) says how to round it. So line 1 - line 21 =
+        # line 11 - line 2 * (1 - line 18), which can fall slightly below 0.
+        share = np.divide(
+            reducible - reduction,
+            reducible,
+            out=np.ones_like(reducible),
+            where=reducible > 0,
+        )
+        # A half rounds up; the offset absorbs float64 error.
+        line_18 = np.floor(share * 1_000 + 0.5 + 1e-9) / 1_000
+        return reduction - iowa_income_tax * (1 - line_18)
diff --git a/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_unit.py b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_unit.py
index f0a8c46af27..43c365ab70f 100644
--- a/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_unit.py
+++ b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_unit.py
@@ -12,33 +12,9 @@ class ia_itemized_deductions_unit(Variable):
         "https://revenue.iowa.gov/media/2650/download?inline",
         "https://revenue.iowa.gov/sites/default/files/2023-01/2022IA1040%2841001%29.pdf",
         "https://revenue.iowa.gov/media/2721/download?inline",
-        "https://revenue.iowa.gov/taxes/tax-guidance/individual-income-tax/iowa-nonconformity-taxpayer-certainty-and-disaster-tax-relief-act-2019",
+        # 2018 IA Schedule A, line 26.
+        "https://revenue.iowa.gov/media/2298/download?inline=#page=1",
     )
     defined_for = StateCode.IA
-
-    def formula(tax_unit, period, parameters):
-        """
-        FROM THE 2021 INSTRUCTIONS (PAGE 46):
-          The $10,000 federal cap on the itemized deduction for state and
-        local taxes does not apply for Iowa purposes. Taxpayers may still
-        deduct eligible state and local taxes paid, independent of the
-        federal dollar limitation.
-        """
-        # compute tax unit's itemized deductions
-        p = parameters(period).gov.states.ia.tax.income.deductions.itemized
-        if p.applies_federal:
-            p_fed = parameters(period).gov.irs.deductions
-            return add(tax_unit, period, p_fed.itemized_deductions)
-        else:
-            deductions = add(
-                tax_unit,
-                period,
-                ["itemized_deductions_less_salt", "real_estate_taxes"],
-            )
-            # Iowa did not adopt the retroactive extension of the mortgage
-            # insurance premium deduction for 2018 and 2019.
-            if p.disallowed_interest_components:
-                deductions = deductions - add(
-                    tax_unit, period, p.disallowed_interest_components
-                )
-            return deductions
+    adds = ["ia_itemized_deductions_pre_limitation"]
+    subtracts = ["ia_itemized_deductions_reduction"]
diff --git a/source_conflicts/candidates.yaml b/source_conflicts/candidates.yaml
index c27bd7db7a0..77c7ef56302 100644
--- a/source_conflicts/candidates.yaml
+++ b/source_conflicts/candidates.yaml
@@ -2555,6 +2555,89 @@ candidates:
   state: ruled
   note: Ruled statute-first in October 2026; the rule requires a model change after confirming that the exclusion
     reaches alternate tax.
+- id: us-ia:itemized-limitation-2018-federal-suspension
+  program: income-tax
+  years:
+    from: 2018
+    to: 2018
+    type: tax
+  question: Did Iowa limit the 2018 itemized deductions of high-income filers, although section 68(f) suspended the
+    federal limitation for 2018 and 701 IAC 41.11 ties the Iowa reduction to the years it applies federally?
+  sources:
+  - kind: statute
+    citation: Iowa Code 422.3(5) (2018)
+    url: https://www.legis.iowa.gov/docs/code/2018/422.3.pdf#page=1
+  - kind: statute
+    citation: Iowa Code 422.9(2) (2018)
+    url: https://www.legis.iowa.gov/docs/code/2018/422.9.pdf#page=1
+  - kind: regulation
+    citation: 701 IAC 41.11 (IAC 2/28/18)
+    url: https://www.legis.iowa.gov/docs/iac/chapter/03-28-2018.701.41.pdf#page=9
+  - kind: statute
+    citation: 2018 Iowa Acts, chapter 1161, sections 58, 63, 66 and 69
+    url: https://www.legis.iowa.gov/docs/acts/2018/CH1161.pdf#page=14
+  - kind: worksheet
+    citation: 2018 IA 104 Iowa Itemized Deductions Worksheet
+    url: https://revenue.iowa.gov/media/2335/download?inline=#page=1
+  readings:
+    statute: For 2018 Iowa Code 422.3(5) defines the Internal Revenue Code as the one in effect on January 1, 2015, so
+      section 68 still limits the deductions Iowa Code 422.9(2) allows. In 2018 Iowa Acts chapter 1161, the legislature
+      moved that date only from 2019 (section 69). For 2018, it applied named sections of Pub. L. 115-97 (section 63)
+      and the section 179 allowance as section 13101 amended it (section 58), none of them section 11045 or 11046. The
+      Iowa Department of Revenue applies the limitation in the 2018 IA 104 and instructions.
+    other: Rule 41.11 applies the reduction "for the same tax years that the provision applies for federal income tax
+      purposes"; read against federal law as amended, which suspends section 68 for 2018, Iowa would not limit 2018
+      deductions.
+  example: A joint filer with federal AGI of $400,000, $32,000 of Schedule A deductions ($4,000 of them medical) and
+    $20,000 of Iowa income tax deducts $30,600 under the IA 104, versus $32,000 with no limitation.
+  pe_us:
+    follows: statute
+    code:
+    - policyengine_us/parameters/gov/states/ia/tax/income/deductions/itemized/limitation/retained.yaml
+    prs:
+    - https://github.com/PolicyEngine/policyengine-us/pull/10101
+  state: unreviewed
+  note: Under Iowa's own definition of the Internal Revenue Code, section 68 applies to 2018, so the rule agrees with
+    the statute and the form.
+- id: us-ia:itemized-limitation-iowa-tax-share-rounding
+  program: income-tax
+  years:
+    from: 2018
+    to: 2018
+    type: tax
+  question: Does Iowa subtract the Iowa income tax at its exact share of the limited deductions, or at the IA 104 line
+    18 percentage rounded to the nearest tenth of a percent?
+  sources:
+  - kind: statute
+    citation: Iowa Code 422.9(2)(a) (2018)
+    url: https://www.legis.iowa.gov/docs/code/2018/422.9.pdf#page=1
+  - kind: regulation
+    citation: 701 IAC 41.5(1) and 41.11(1) (IAC 2/28/18)
+    url: https://www.legis.iowa.gov/docs/iac/chapter/03-28-2018.701.41.pdf#page=10
+  - kind: worksheet
+    citation: 2018 IA 104 Iowa Itemized Deductions Worksheet, line 18
+    url: https://revenue.iowa.gov/media/2335/download?inline=#page=1
+  readings:
+    statute: Iowa Code 422.9(2)(a) subtracts "the deduction for Iowa income taxes" and doesn't say how much of it stays
+      deducted after the overall reduction. Rule 41.11(1) requires high-income filers to complete Form 41-104. The copy
+      of the form it prints gives the share with no rounding instruction (line 6), and rule 41.5(1) subtracts the tax
+      "to the extent" included.
+    other: The IA 104 enters line 15 divided by line 17 "to the nearest tenth of a percent" before applying it to the
+      Iowa income tax.
+  example: A joint filer with federal AGI of $400,000, $30,000 of Schedule A deductions ($4,000 of them medical) and
+    $20,000 of Iowa income tax enters 94.8% and deducts $28,640; the exact share, 94.78%, gives $28,643.48.
+  pe_us:
+    follows: gap-fill
+    code:
+    - policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_itemized_deductions_reduction.py
+    prs:
+    - https://github.com/PolicyEngine/policyengine-us/pull/10101
+  state: unreviewed
+  note: Rounding moves the result by at most 0.05% of the Iowa income tax, but it can raise line 21 above line 1. A
+    joint filer at $500,000 with only $20,000 of medical expenses and investment interest and $9,700 of Iowa income
+    tax enters 44.3% and gets $20,002.90; the exact share gives $20,000. The statute and the rule's copy of the form
+    leave the rounding open, and the rule requires the form, so the model follows the 2018 form. The form gives no tie
+    rule, so the model rounds a half up, and a 93.25% share enters as 93.3%.
 - id: us-ia:low-income-exemption-claimant-income
   program: income-tax
   years: