How we build calculators

Free UK Tools & Calculators publishes free, browser-based estimators for everyday UK money questions — mortgages, stamp duty, take-home pay, National Insurance and similar. This page explains how those tools are put together so you can judge what to trust and what to check elsewhere.

What a result on this site 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 personalised advice. If a payslip, solicitor completion statement or official calculator disagrees with us, treat the official document as correct.

Where the rates come from

Money tools use commonly published UK structures: Income Tax and National Insurance thresholds, Capital Gains Tax rates and the annual exempt amount, residential stamp duty-style bands by nation, statutory redundancy weekly caps, VAT rates, and similar public figures. Each finance tool lists official sources (usually GOV.UK, HMRC, Revenue Scotland, the Welsh Revenue Authority or MoneyHelper) and shows a “rates & guidance reviewed” date.

We do not scrape other websites for copy. Introductions, worked examples and written guides are written for this site.

The formulae are industry-standard mathematics, not a proprietary black box. Examples include the standard repayment-mortgage amortisation; credit-card payoff as monthly compound interest at APR ÷ 12; fuel as miles ÷ UK MPG × 4.54609 litres; VAT as net × (1 + rate) to add and gross ÷ (1 + rate) to remove; inflation as amount × (1 + i)^n; and savings compound interest as a lump plus an ordinary annuity.

When we update

The UK tax year changes on 6 April. We aim to refresh affected thresholds and notes around that date, and sooner if a government announcement changes a rate we display (for example a stamp duty holiday or a National Insurance cut). Developer and everyday utilities change only when the product itself changes.

Frozen thresholds still get a review pass so the “last reviewed” date stays honest even when the numbers have not moved.

What we deliberately leave out

Real tax and lending have edges a short web tool should not pretend to cover. Typical omissions — called out on the relevant page — include unusual tax codes, benefits-in-kind, directors’ National Insurance, lifetime Inheritance Tax gifts, interest-only mortgages, product fees, and the legal tests for first-time buyer or main-home replacement relief. Showing a clean number that ignores those edges would be more misleading than a labelled estimate.

Privacy of the numbers you type

Interactive tools run in your browser with client-side JavaScript. We do not operate a database of salaries, loan amounts or JSON you paste. Hosting logs may record that a page was requested (normal for any website). See the Privacy Policy for cookies and advertising.

How to report a mistake

If a worked example does not match the form, or a rate looks stale after a Budget, email freeuktools@aivora.digital or use the contact page. We cannot give personal financial advice in reply, but we do correct tool errors.