The calculator uses one centrally versioned payroll-2026-27 dataset, reviewed against the cited GOV.UK publications on 20 September 2026. The calculation engine is pure TypeScript: the interface supplies inputs and verified rates, while the engine returns annual, monthly and weekly breakdowns without a network request.
England, Wales and Northern Ireland use 20%, 40% and 45% employment-income bands after the effective allowance. Scotland uses the published starter, basic, intermediate, higher, advanced and top bands. Employee and employer National Insurance are calculated independently for the selected category.