How we build calculators
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Free UK Tools publishes free calculators for everyday UK money questions, such as mortgages, stamp duty, take-home pay and National Insurance. This page explains how they are put together, so you can judge what to rely on and what to check elsewhere.
What a result is
A result is a simplified estimate from a published formula or band table. It is not a lender illustration, an HMRC calculation, a legal opinion or personal advice. If a payslip, a solicitor’s completion statement or an official calculator disagrees with us, treat the official figure as correct.
Where the rates come from
The money calculators use published UK figures: Income Tax and National Insurance thresholds, Capital Gains Tax rates and the annual exempt amount, stamp duty bands for each nation, the statutory redundancy weekly cap, VAT rates and similar. Each calculator lists its official sources (usually GOV.UK, HMRC, Revenue Scotland, the Welsh Revenue Authority or MoneyHelper) and shows the date its rates were last checked against them.
The introductions, worked examples and guides are written by Aivora Digital for this site. We do not copy text from other websites.
The formulas are standard. Examples: a repayment mortgage uses the usual amortisation formula; credit card payoff charges monthly interest at the APR ÷ 12; fuel cost is miles ÷ UK mpg × 4.54609 litres × the price per litre; VAT is added with net × (1 + rate) and removed with gross ÷ (1 + rate); inflation uses amount × (1 + rate)^years; and savings growth is the starting sum plus regular contributions, both compounded.
When we update
The UK tax year starts on 6 April. We update the affected thresholds and notes around that date, and sooner if the government changes a rate we use, for example at a Budget. Everyday calculators such as the percentage calculator only change when we improve the calculator itself.
When a threshold is frozen, we still check it each year and update the “rates checked” date, so the date always reflects a real check. Every page also shows a “Last updated” date, which changes whenever the page itself changes.
What we leave out
Real tax and lending have complications a short calculator cannot cover well. Common examples, each mentioned on the relevant page, include unusual tax codes, benefits in kind, directors’ National Insurance, lifetime gifts for Inheritance Tax, interest-only mortgages, product fees, and the legal tests for first-time buyer relief or replacing a main home. A clearly labelled estimate is more useful than a precise-looking number that quietly ignores them.
The numbers you type
The calculators work in your browser. We do not keep a record of the salaries, prices or loan amounts you enter. Like any website, our host logs that a page was requested. The privacy policy covers cookies, analytics and advertising.
Reporting a mistake
If a worked example does not match the calculator, or a rate looks out of date after a Budget, email freeuktools@aivora.digital or use the contact page. We cannot give personal financial advice, but we do check every report against the official source and correct the page if we got it wrong.