Income Tax uses the versioned income-tax-2026-27 rate set and the same engine as the Income Tax Calculator. National Insurance and the student loan thresholds come from the payroll-2026-27 rate set used by the Payroll and National Insurance calculators, so no second copy of any rate exists in this project.
The calculation is pure TypeScript running in your browser, with full precision held internally and rounding applied only when a figure is displayed. Unit tests cover the Personal Allowance boundary, the National Insurance thresholds, each student loan plan, the postgraduate loan, all three pension methods and the £100,000 to £125,140 taper.