Our methodology

Every Calcsmith calculator follows the same three-step build process: source the formulas from authoritative public data, implement them as pure TypeScript functions with full test coverage, and verify the outputs against worked examples published by HMRC or relevant regulators.

We document the assumptions on each tool page so you can see exactly what's being calculated. Where rounding rules differ between official guidance and intuitive display, we follow the official rule and note the difference.

Primary data sources

• HMRC PAYE manuals and rate cards (income tax, National Insurance, dividend tax, CGT)
• HM Land Registry SDLT rate tables (Stamp Duty Land Tax)
• Revenue Scotland (LBTT) and Welsh Revenue Authority (LTT)
• Department for Work & Pensions (state pension, auto-enrolment thresholds)
• The Pensions Regulator and FCA published guidance
• DVLA published VED (road tax) rates

Annual updates

The full rate set is reviewed every April when the new UK tax year begins, with mid-year updates whenever the Treasury announces changes (Spring Budget, Autumn Statement). Updates are published before they take legal effect so you can plan ahead.

Verification

Each calculator is verified against worked examples in HMRC manuals where available. Maths is performed in integer pence to avoid floating-point drift, ensuring identical results across browsers and devices.