diff --git a/changelog.d/2517.fixed.md b/changelog.d/2517.fixed.md
new file mode 100644
index 0000000000..ce59fe4bc0
--- /dev/null
+++ b/changelog.d/2517.fixed.md
@@ -0,0 +1 @@
+- Carer's Allowance earnings read their pension contribution deduction share (one-half, Computation of Earnings Regulations 1996 regs. 10(4)(b) and 13) from a new parameter, `gov.dwp.carers_allowance.pension_contribution_deduction_rate`, instead of a hard-coded 0.5, so the share can be reformed.
diff --git a/policyengine_uk/parameters/gov/dwp/carers_allowance/pension_contribution_deduction_rate.yaml b/policyengine_uk/parameters/gov/dwp/carers_allowance/pension_contribution_deduction_rate.yaml
new file mode 100644
index 0000000000..0f6b963c1e
--- /dev/null
+++ b/policyengine_uk/parameters/gov/dwp/carers_allowance/pension_contribution_deduction_rate.yaml
@@ -0,0 +1,17 @@
+description: Carer's Allowance deducts this share of pension contributions when it calculates the net earnings and net profit that the earnings limit tests.
+values:
+  1996-11-25: 0.5
+metadata:
+  period: year
+  unit: /1
+  label: Carer's Allowance earnings pension contribution deduction rate
+  economy: false
+  reference:
+    - title: The Social Security Benefit (Computation of Earnings) Regulations 1996, regulation 10(4)(b) (employed earners)
+      href: https://www.legislation.gov.uk/uksi/1996/2745/regulation/10
+    - title: The Social Security Benefit (Computation of Earnings) Regulations 1996, regulation 13(1)(b)(ii), (4)(c) and (10)(b) (self-employed earners)
+      href: https://www.legislation.gov.uk/uksi/1996/2745/regulation/13
+    - title: The Social Security Benefit (Computation of Earnings) Regulations (Northern Ireland) 1996, regulation 10(4)(b) (employed earners)
+      href: https://www.legislation.gov.uk/nisr/1996/520/regulation/10
+    - title: The Social Security Benefit (Computation of Earnings) Regulations (Northern Ireland) 1996, regulation 13(1)(b)(ii), (4)(c) and (10)(b) (self-employed earners)
+      href: https://www.legislation.gov.uk/nisr/1996/520/regulation/13
diff --git a/policyengine_uk/tests/policy/baseline/gov/dwp/carers_allowance_pension_contribution_deduction_rate.yaml b/policyengine_uk/tests/policy/baseline/gov/dwp/carers_allowance_pension_contribution_deduction_rate.yaml
new file mode 100644
index 0000000000..723a7a972b
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/dwp/carers_allowance_pension_contribution_deduction_rate.yaml
@@ -0,0 +1,123 @@
+# Carer's Allowance net earnings deduct one-half of pension contributions
+# (Computation of Earnings Regulations 1996 reg. 10(4)(b)) and of pension
+# premiums from trading profit (regs. 13(1)(b)(ii), (4)(c) and (10)(b)). The
+# share is gov.dwp.carers_allowance.pension_contribution_deduction_rate. Rates
+# other than 0.5 below are model reforms, not current law.
+#
+# 2026-27: personal allowance, NI primary threshold and Class 4 lower profits
+# limit £12,570; Class 4 main rate 6%; basic rate 20%. The earnings limit is
+# £204 a week (£10,608 a year) and Carer's Allowance is £86.45 a week. Pay of
+# £12,000 is under the allowance and the threshold, so the earnings test
+# deducts no tax (it takes PAYE on the pay alone) and no Class 1.
+
+- name: At current law Carer's Allowance earnings deduct half of pension contributions
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    employment_income: 12_000
+    personal_pension_contributions: 3_000
+    care_hours: 40
+    country: ENGLAND
+  output:
+    # 12,000 - 0.5 x 3,000 = 10,500, or £201.92 a week: under the limit.
+    pension_contributions: 3_000
+    ni_class_1_employee: 0
+    carers_allowance_earnings: 10_500
+    is_gainfully_employed_for_carers_allowance: false
+    carers_allowance: 86.45 * 52
+
+- name: A lower share raises earnings by contributions times the change and ends entitlement
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    # Model reform, not current law.
+    gov.dwp.carers_allowance.pension_contribution_deduction_rate: 0.25
+    age: 45
+    employment_income: 12_000
+    personal_pension_contributions: 3_000
+    care_hours: 40
+    country: ENGLAND
+  output:
+    # 12,000 - 0.25 x 3,000 = 11,250, or £216.35 a week: over the limit.
+    # That is 750 more than at half: 3,000 x (0.5 - 0.25).
+    carers_allowance_earnings: 11_250
+    is_gainfully_employed_for_carers_allowance: true
+    carers_allowance_pre_overlap: 0
+    carers_allowance: 0
+
+- name: At current law a carer with smaller contributions is over the earnings limit
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    employment_income: 12_000
+    personal_pension_contributions: 2_000
+    care_hours: 40
+    country: ENGLAND
+  output:
+    # 12,000 - 0.5 x 2,000 = 11,000, or £211.54 a week: over the limit.
+    carers_allowance_earnings: 11_000
+    is_gainfully_employed_for_carers_allowance: true
+    carers_allowance: 0
+
+- name: A higher share lowers earnings by contributions times the change and restores entitlement
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    # Model reform, not current law.
+    gov.dwp.carers_allowance.pension_contribution_deduction_rate: 0.75
+    age: 45
+    employment_income: 12_000
+    personal_pension_contributions: 2_000
+    care_hours: 40
+    country: ENGLAND
+  output:
+    # 12,000 - 0.75 x 2,000 = 10,500, or £201.92 a week: under the limit.
+    # That is 500 less than at half: 2,000 x (0.75 - 0.5).
+    carers_allowance_earnings: 10_500
+    is_gainfully_employed_for_carers_allowance: false
+    carers_allowance: 86.45 * 52
+
+- name: A lower share raises trading earnings by premiums times the change
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    # Model reform, not current law.
+    gov.dwp.carers_allowance.pension_contribution_deduction_rate: 0.4
+    age: 45
+    self_employment_income: 15_000
+    personal_pension_contributions: 7_700
+    care_hours: 40
+    country: ENGLAND
+  output:
+    # Notional tax 20% x 2,430 = 486 and Class 4 6% x 2,430 = 145.80 (reg.
+    # 14). At half: 15,000 - 486 - 145.80 - 3,850 = 10,518.20, or £202.27 a
+    # week, under the limit. At 0.4: 15,000 - 486 - 145.80 - 3,080 =
+    # 11,288.20, or £217.08 a week, over it: 770 more, 7,700 x (0.5 - 0.4).
+    carers_allowance_earnings: 11_288.20
+    is_gainfully_employed_for_carers_allowance: true
+    carers_allowance: 0
+
+- name: A reform of the Housing Benefit share leaves Carer's Allowance earnings
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    # Model reform, not current law.
+    gov.dwp.housing_benefit.means_test.pension_contribution_deduction_rate: 0.25
+    age: 45
+    employment_income: 12_000
+    personal_pension_contributions: 2_000
+    care_hours: 40
+    country: ENGLAND
+  output:
+    # The carer is over the limit, so no Carer's Allowance is paid and the
+    # £12,000 of pay, less relief on the contributions, is the only income:
+    # no income tax. Housing Benefit net earnings: 12,000 - 0.25 x 2,000 =
+    # 11,500. Carer's Allowance keeps its own half: 12,000 - 0.5 x 2,000 =
+    # 11,000, still £211.54 a week.
+    income_tax: 0
+    housing_benefit_net_earnings: 11_500
+    carers_allowance_earnings: 11_000
+    is_gainfully_employed_for_carers_allowance: true
+    carers_allowance: 0
diff --git a/policyengine_uk/tests/test_carers_allowance_pension_contribution_deduction_rate_properties.py b/policyengine_uk/tests/test_carers_allowance_pension_contribution_deduction_rate_properties.py
new file mode 100644
index 0000000000..be9c21dea3
--- /dev/null
+++ b/policyengine_uk/tests/test_carers_allowance_pension_contribution_deduction_rate_properties.py
@@ -0,0 +1,194 @@
+"""Property-based tests for Carer's Allowance's pension contribution share.
+
+The Carer's Allowance earnings limit tests net earnings under the Social
+Security Benefit (Computation of Earnings) Regulations 1996, which deduct one
+half of pension contributions from an employed earner's net earnings (reg.
+10(4)(b)) and one half of pension premiums from a self-employed earner's net
+profit (regs. 13(1)(b)(ii), (4)(c) and (10)(b)). Northern Ireland's 1996
+Regulations (SR 1996/520) say the same. The model reads the share from
+gov.dwp.carers_allowance.pension_contribution_deduction_rate.
+
+Invariants, for any generated population of working-age carers in any UK
+country and year, with pay, trading profit (a loss counts as nil) and
+employee and personal pension contributions of at most 30% of earnings, so
+that no floor binds:
+
+1. Exactness: when the share moves from 0.5 to r, carers_allowance_earnings
+   falls by exactly the person's contributions x (r - 0.5).
+2. Locality: the Housing Benefit share is reformed at the same time to a
+   different value, so earnings that read Housing Benefit's share would miss
+   invariant 1.
+3. Eligibility follows earnings through the limit: on both sides the person
+   is gainfully employed exactly when weekly earnings, with a fraction of a
+   penny disregarded (reg. 5), exceed the limit, and then has no underlying
+   entitlement. The pence are counted in the precision the model stores
+   earnings in (float32), as the formula counts them. A lower share can only end entitlement and a higher share can
+   only restore it.
+4. Current law: the share is 0.5 in every year from 2015 (the first year the
+   model simulates) to 2030.
+"""
+
+import numpy as np
+from hypothesis import HealthCheck, example, given, settings
+from hypothesis import strategies as st
+
+from policyengine_uk import Simulation
+from policyengine_uk.system import system
+
+CA_SHARE = "gov.dwp.carers_allowance.pension_contribution_deduction_rate"
+HB_SHARE = "gov.dwp.housing_benefit.means_test.pension_contribution_deduction_rate"
+# Every period the model simulates.
+ALL_YEARS = "2000-01-01.2100-12-31"
+COUNTRIES = ["ENGLAND", "WALES", "SCOTLAND", "NORTHERN_IRELAND"]
+YEARS = [2016, 2020, 2024, 2026, 2030]
+# Shares on a grid of twentieths, each exact in binary.
+SHARES = [i / 20 for i in range(21) if i != 10]
+# The model stores amounts as float32, whose spacing is about 0.008 at the
+# largest earnings drawn (up to 120,000), so two stored values can differ from
+# the exact difference by more than a penny.
+TOLERANCE = 0.05
+
+PROPERTY_SETTINGS = settings(
+    max_examples=6,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+
+
+@st.composite
+def carers(draw):
+    pay = draw(st.floats(0, 60_000, allow_nan=False))
+    profit = draw(st.floats(-20_000, 60_000, allow_nan=False))
+    earnings = pay + max(0.0, profit)
+    # Net earnings before the pension deduction are more than 60% of
+    # earnings in every year and country tested (about 70% at the top of
+    # the range), so contributions of at most 30% leave them positive at
+    # any share up to one.
+    employee_share = draw(st.one_of(st.just(0.0), st.floats(0, 0.15)))
+    personal_share = draw(st.one_of(st.just(0.0), st.floats(0, 0.15)))
+    return dict(
+        country=draw(st.sampled_from(COUNTRIES)),
+        age=draw(st.integers(25, 60)),
+        employment_income=pay,
+        self_employment_income=profit,
+        employee_pension_contributions=round(pay * employee_share, 2),
+        personal_pension_contributions=round(earnings * personal_share, 2),
+    )
+
+
+@st.composite
+def reforms(draw):
+    ca, hb = draw(
+        st.lists(st.sampled_from(SHARES), min_size=2, max_size=2, unique=True)
+    )
+    return dict(ca=ca, hb=hb)
+
+
+def situation(people, year):
+    persons, benunits, households = {}, {}, {}
+    for i, carer in enumerate(people):
+        name = f"carer_{i}"
+        persons[name] = {
+            key: {year: value} for key, value in carer.items() if key != "country"
+        }
+        persons[name]["care_hours"] = {year: 40}
+        benunits[f"b{i}"] = {"members": [name]}
+        households[f"h{i}"] = {
+            "members": [name],
+            "country": {year: carer["country"]},
+        }
+    return {"people": persons, "benunits": benunits, "households": households}
+
+
+VARIABLES = [
+    "pension_contributions",
+    "carers_allowance_earnings",
+    "is_gainfully_employed_for_carers_allowance",
+    "carers_allowance_pre_overlap",
+]
+
+
+def calculate(people, year, reform=None):
+    sim = Simulation(situation=situation(people, year), reform=reform)
+    # Keep each variable's stored dtype: the rounding check below must count
+    # pence in the precision the formula does.
+    return {v: np.asarray(sim.calculate(v, year)) for v in VARIABLES}
+
+
+def over_limit(earnings, limit):
+    # Reg. 5: a fraction of a penny is disregarded. ``earnings`` is the stored
+    # array, so this counts the pence as is_gainfully_employed_for_carers_allowance
+    # does, in float32: promoting to float64 first floors 10,608.52 a year,
+    # stored as 10,608.51953125, to 204.00 a week instead of 204.01.
+    return np.floor(earnings / 52 * 100 + 1e-6) / 100 > limit
+
+
+# A carer one penny a week over the 2026-27 limit (10,608.52 / 52 = 204.01), on
+# the float32 boundary described in over_limit.
+PENNY_OVER = dict(
+    country="ENGLAND",
+    age=45,
+    employment_income=10_608.52,
+    self_employment_income=0.0,
+    employee_pension_contributions=0.0,
+    personal_pension_contributions=0.0,
+)
+
+
+@PROPERTY_SETTINGS
+@given(
+    people=st.lists(carers(), min_size=1, max_size=24),
+    shares=reforms(),
+    year=st.sampled_from(YEARS),
+)
+@example(people=[PENNY_OVER], shares=dict(ca=0.25, hb=0.75), year=2026)
+def test_earnings_fall_by_contributions_times_the_change_in_share(people, shares, year):
+    base = calculate(people, year)
+    moved = calculate(
+        people,
+        year,
+        {
+            CA_SHARE: {ALL_YEARS: shares["ca"]},
+            HB_SHARE: {ALL_YEARS: shares["hb"]},
+        },
+    )
+    limit = system.parameters(f"{year}-01-01").gov.dwp.carers_allowance.earnings_limit
+    for i, carer in enumerate(people):
+        case = (carer, shares, year)
+        contributions = (
+            carer["employee_pension_contributions"]
+            + carer["personal_pension_contributions"]
+        )
+        stored = float(base["pension_contributions"][i])
+        assert abs(stored - contributions) < TOLERANCE, case
+        before = float(base["carers_allowance_earnings"][i])
+        after = float(moved["carers_allowance_earnings"][i])
+        if contributions > 0:
+            # Floors are slack: earnings stay positive on both sides.
+            assert before > 0 and after > 0, case
+        # Invariants 1 and 2: exactly contributions x the change in the
+        # Carer's Allowance share, not the Housing Benefit share.
+        change = before - after
+        assert abs(change - contributions * (shares["ca"] - 0.5)) < TOLERANCE, case
+        # Invariant 3: the gainful-employment test follows earnings.
+        for values in (base, moved):
+            over = over_limit(values["carers_allowance_earnings"][i], limit)
+            gainful = bool(values["is_gainfully_employed_for_carers_allowance"][i])
+            assert gainful == over, case
+            if gainful:
+                assert values["carers_allowance_pre_overlap"][i] == 0, case
+        was = bool(base["is_gainfully_employed_for_carers_allowance"][i])
+        now = bool(moved["is_gainfully_employed_for_carers_allowance"][i])
+        if shares["ca"] < 0.5:
+            assert now or not was, case
+        else:
+            assert was or not now, case
+
+
+def test_the_share_is_one_half_in_every_year():
+    for year in range(2015, 2031):
+        for day in ("01-01", "04-30"):
+            parameters = system.parameters(f"{year}-{day}")
+            share = parameters.gov.dwp.carers_allowance
+            assert share.pension_contribution_deduction_rate == 0.5, (year, day)
diff --git a/policyengine_uk/variables/gov/dwp/carers_allowance_earnings.py b/policyengine_uk/variables/gov/dwp/carers_allowance_earnings.py
index b49b6618d0..e3dc980d29 100644
--- a/policyengine_uk/variables/gov/dwp/carers_allowance_earnings.py
+++ b/policyengine_uk/variables/gov/dwp/carers_allowance_earnings.py
@@ -9,8 +9,10 @@ class carers_allowance_earnings(Variable):
     documentation = (
         "Net earnings from employment and self-employment, as the Carer's "
         "Allowance gainful-employment test counts them (Computation of "
-        "Earnings Regulations 1996), less half of the person's pension "
-        "contributions. A trading loss counts as nil.\n\n"
+        "Earnings Regulations 1996), less a share of the person's pension "
+        "contributions: one half in law (regs. 10(4)(b) and 13(4)(c)), read "
+        "from gov.dwp.carers_allowance.pension_contribution_deduction_rate. "
+        "A trading loss counts as nil.\n\n"
         "Employment (reg. 10(4)): gross pay less the income tax and primary "
         "Class 1 contributions deducted from it. The law deducts the tax "
         "actually withheld; the model approximates it as the tax PAYE would "
@@ -119,7 +121,12 @@ def formula(person, period, parameters):
             * class_2_contribution_weeks(period.start.year)
         )
 
-        half_pension_contributions = 0.5 * person("pension_contributions", period)
+        # Regs. 10(4)(b) and 13(1)(b)(ii), (4)(c) and (10)(b): one half of
+        # pension contributions and premiums.
+        pension_share = parameters(
+            period
+        ).gov.dwp.carers_allowance.pension_contribution_deduction_rate
+        pension_deduction = pension_share * person("pension_contributions", period)
         return max_(
             0,
             pay
@@ -129,5 +136,5 @@ def formula(person, period, parameters):
             - trading_tax
             - trading_class_4
             - trading_class_2
-            - half_pension_contributions,
+            - pension_deduction,
         )