TRANSPARENCY

Sources and methodology

Every calculation should show where its rules came from, when they were checked and what the result does not include.

Update policy

Official sources are monitored on the schedule below. A detected change to a critical tax source stops automatic publication and requires a rules review plus boundary tests before a new version is released.

Income TaxGOV.UK and HMRCDaily

National InsuranceGOV.UK and HMRCDaily

Scottish Income TaxGOV.UK and Scottish Government publicationsDaily

Student loan repaymentsGOV.UK and Student Loans Company guidanceDaily

UK bank holidaysGOV.UK bank holiday dataWeekly

Council Tax statisticsDLUHC and devolved administration datasetsWeekly

Calculation review

Each ruleset is versioned by tax year. Tests cover values below, at and above important thresholds, plus comparisons with published official examples where available. Estimates are rounded only for display; calculations retain full precision.

Current UK ruleset: GB-2026.27.1 · Last reviewed: 14 July 2026.

Corrections

If a source changes or an error is found, affected calculators are reviewed together. Material corrections update the rule version and displayed review date so users can identify the calculation basis.