Our approach
CalcBudget calculators are designed to give a clear, realistic estimate for common UK money questions and to show how that estimate was reached. We prefer simple, transparent formulas that you can check by hand over complicated models that hide their assumptions. Each calculator page explains its method, gives worked examples and lists what it does not cover.
Where the figures come from
Where a calculation depends on a tax rule, statutory threshold or official definition, we use primary sources, mainly GOV.UK and HMRC. Examples include Income Tax rates and the Personal Allowance, employee National Insurance thresholds, Stamp Duty Land Tax bands and reliefs, and VAT rates. Each calculator lists the exact source pages it uses, and our sources page collects them in one place. For background on interest rates and inflation we refer to the Bank of England and the Office for National Statistics.
Calculators that do not depend on tax rules, such as the loan, compound interest and savings calculators, use standard financial formulas. For example, the loan calculator uses the standard amortisation formula for a fixed-rate loan repaid in equal monthly instalments, and the compound interest calculator applies one-twelfth of the annual rate each month.
How calculations run
All calculators run in your web browser. The figures you enter are used on your device to produce the result and are not sent to us or stored. Results are rounded to the nearest penny for display.
Testing
Before a calculator is published or changed, we:
- check each rate and threshold against the current GOV.UK or HMRC page;
- run test cases across the range of likely inputs, including values just below and above each threshold, where errors are most likely;
- compare results with worked examples published on GOV.UK where they exist, such as the Stamp Duty examples, and with calculations done by hand;
- check the worked examples shown on the page by running them through the calculator itself.
Reviews and updates
UK tax rates and allowances usually change at the start of the tax year on 6 April. We review every tax-related calculator each April, and when changes are announced at other times, for example in the Budget. Each tax-related calculator states the tax year it uses and when it was last reviewed. When a rule changes part-way through a year, we update the calculator from the date the change takes effect and note it on the page.
Assumptions and limits
To keep calculators clear, each one makes simplifying assumptions, which are listed on the page. Typical examples are that a rate stays fixed, that payments are made monthly, or that a standard tax code applies. Some situations are deliberately not modelled, such as Scottish Income Tax bands in the take-home pay calculator, or Scotland and Wales in the Stamp Duty calculator. Where a situation is outside a calculator's scope, the page says so.
Results are estimates
Calculator results are estimates for planning. They are not financial, tax or mortgage advice and are not a substitute for an official calculation, a lender's illustration, a payslip or a solicitor's completion statement. If you think a result is wrong, please tell us using the details on our contact page.
Last updated October 2026.