Methodology

UK Finance & Accountancy Tools publishes calculators and guides that depend on current UK tax and financial rules. This page explains how those figures are sourced, how calculations are built and tested, and how we handle errors — so you can judge the reliability of the numbers.

Where the data comes from

All tax thresholds, rates and allowances are stored in a single, central data layer rather than being duplicated inside individual calculators. This keeps every tool consistent and makes annual updates reliable. The figures are taken from official sources, primarily GOV.UK and HMRC publications for the relevant tax year, plus MoneyHelper, the Bank of England and the FCA where those bodies set the relevant rule or rate.

Where a rule varies by region — such as Scottish income tax or Welsh stamp duty — the calculator either supports the variation or states the assumption it makes. Every calculator page links to the specific official source it relies on.

How rules are updated

Rates are reviewed at the start of each UK tax year (6 April) and whenever an emergency Budget or Finance Act changes mid-year rules. Each page displays a last-reviewed date so you can see when its figures were checked. The current tax year supported is 2026/27.

Rounding and precision

Calculators compute results using the rules that apply to the tax year selected, then round money values to whole pence or pounds where appropriate. Where a result depends on rounding (for example, monthly PAYE figures versus an annual figure), the calculator shows the result that the relevant method produces and states its assumption.

How calculations are built

Each calculator is defined as data — its inputs, output rows, formula and assumptions — and the calculation logic is kept separate from the page layout. This separation means a single bug in a formula is fixed once and applied everywhere, and the same logic is reused across the full breakdown and any related tools.

How calculations are tested

Every financial formula is covered by automated unit tests. These check edge cases such as zero values, values just below and just above a threshold, exact threshold boundaries, large values, invalid inputs, rounding behaviour and switching between tax years. We do not deploy a change to a calculator while its financial tests are failing.

Tax-year versioning

Figures are versioned by tax year rather than by calendar date. When a new tax year's rules are verified, they are added alongside the previous year so comparisons and switching work correctly. The default is the current tax year.

Reporting errors

If you spot an error, please tell us. We assess every report, correct verified mistakes quickly, and update the relevant page. See our corrections policy for how reports are handled.