How we check our calculators
Every Calcistry calculator shows its method and sources on the page. This is the process behind them: where the figures come from, how they are checked, and what we deliberately leave out.
Last updated:
Sources for UK calculators
UK tax rates, bands, allowances and thresholds come only from official sources:
- GOV.UK and HMRC — Income Tax, National Insurance, tax codes, student loans, pensions, ISAs, Capital Gains Tax, dividend and savings tax, and Stamp Duty Land Tax (England and Northern Ireland).
- gov.scot — Scottish Income Tax rates and bands, including the Scottish Government's technical factsheet.
- Revenue Scotland — Land and Buildings Transaction Tax (LBTT), first-time buyer relief and the Additional Dwelling Supplement.
- Welsh Revenue Authority — Land Transaction Tax (LTT) rates and higher rates in Wales, published on gov.wales.
- legislation.gov.uk — the underlying legislation, where a rule needs to be read at source.
Each calculator links the exact pages it relies on in its Method & sources box, with the date they were checked.
Sources for US calculators
Our US calculators (loans, mortgages, compounding, debt payoff, dividends and FIRE) are money-maths tools. They don't calculate US tax, so they don't rely on tax-authority figures: they use standard financial formulas such as loan amortisation and compound growth, and each page shows the formula it uses. Where a figure depends on your own plan or lender, such as a 401(k) match or a recast fee, the page says so and asks you to confirm it.
Checked against official worked examples
Before a UK tax calculator is published, we run the official worked examples through it and check that it gives the same answer. Where GOV.UK, HMRC, gov.scot, Revenue Scotland or the Welsh Revenue Authority publish an example (for instance a National Insurance or student loan calculation, a pension tax relief example or a Stamp Duty calculation), that example becomes part of the calculator's checks. If our result differs, the calculator doesn't go live until we find out why.
Automated tests before every update
Each calculation engine has its own set of automated tests: the official examples, the rates and thresholds themselves and the points where one band ends and the next begins, and for many calculators an independent recalculation of the same result by a different method. The full set runs before every update to the site, and an update is not published unless every test passes.
Annual review
UK tax figures change at the start of each tax year on 6 April, and are usually announced at the UK Budget beforehand. After each Budget, and again when the new tax year starts, we review every UK calculator against the updated official sources, update the figures and tests, and change the Reviewed date on the page. Scottish and Welsh rates are checked separately, since they are set by their own governments. If a rule changes mid-year, we update the calculator then.
Rounding and assumptions
- Full precision inside, rounded on screen. Calculations run at full precision. Results are usually shown to the nearest pound (or dollar), and to the penny where that matters, such as a monthly mortgage payment. Because of this, a total can differ by £1 from the sum of the rounded parts.
- Official rounding where it exists. Where official guidance specifies rounding, for example rounding a monthly student loan repayment down to the pound, we follow it.
- Assumptions are stated. Each calculator lists its assumptions on the page, such as a whole-year view of pay rather than week-by-week payroll, or a constant growth rate. Default inputs are illustrative, not forecasts.
- The tax year is shown. UK calculators say which tax year their figures apply to.
What's out of scope
- Personal financial, tax or investment advice, and recommendations of products or providers.
- Every individual circumstance. Each calculator lists the cases it doesn't model; for those, check with GOV.UK, HMRC, your payroll team or a qualified adviser.
- Forecasts. Growth rates, interest rates and returns are inputs you choose, not predictions.
Found something that looks wrong? Email hello@calcistry.com with the page and the numbers you used. How we handle corrections is set out in our editorial policy.