Methodology

This page explains how Credibrate calculators are built, where the underlying data comes from, how calculations are verified, how often content is updated, and how affiliate relationships are managed — so that the numbers you see can be understood, scrutinised, and trusted.

Data sources

Every calculator on this site uses published, verifiable data from official sources. No proprietary models or estimated figures are used without disclosure. The primary sources by calculator type are:

• Income tax, National Insurance, and salary: HMRC tax rates and allowances, published annually at gov.uk/tax-rates-and-thresholds. Scottish income tax rates are sourced from the Scottish Government budget and confirmed against gov.scot.
• Stamp duty (SDLT, LBTT, LTT): HMRC Stamp Duty Land Tax guidance for England and Northern Ireland, Revenue Scotland for LBTT, and the Welsh Revenue Authority for LTT.
• Pensions: HMRC pension tax relief rules, annual allowance guidance published at gov.uk/tax-on-your-private-pension, and MoneyHelper pension guidance from the Money and Pensions Service.
• ISAs and savings: HMRC ISA rules and allowances (gov.uk/individual-savings-accounts), NS&I Premium Bonds prize fund rate, and Bank of England base rate data.
• Mortgage: Standard mortgage amortisation mathematics. Affordability multiple and stress test methodology follows FCA Mortgage Conduct of Business (MCOB) rules and Financial Policy Committee (FPC) guidance.
• VAT and business: HMRC VAT rates (gov.uk/vat-rates) and HMRC capital allowances rates for depreciation calculators.
• Health: NHS BMI categories, Mifflin-St Jeor equation for basal metabolic rate, Harris-Benedict activity multipliers, and NHS guidance on healthy ranges where applicable.
• Unit conversions: SI definitions from the International Bureau of Weights and Measures (BIPM) and NIST where applicable.

How calculators are built

Calculator logic is implemented as pure functions in TypeScript. No external APIs are called at runtime, and no user data is sent to a server. Every calculation happens entirely in the browser. The underlying code is tested to 100% coverage against known inputs and expected outputs before deployment.

Complex rules — income tax with personal allowance tapering, Scottish rate divergence, National Insurance thresholds, stamp duty banding — are modelled from first principles using published legislation rather than approximations. Where simplifying assumptions are made, they are stated on the relevant calculator page.

Calculation precision

To avoid floating-point rounding errors that compound across multi-step calculations, all monetary values are stored and computed as integers in pence. A salary of £45,000 is represented as 4,500,000 pence throughout the calculation engine and only converted to a display value at the final step. Interest rates and percentages are stored as basis points: 4.25% is stored as 425. This approach eliminates the class of errors where, for example, 0.1 + 0.2 does not equal 0.3 in floating-point arithmetic.

Where an official HMRC calculator or equivalent exists — the HMRC PAYE calculator for take-home pay, the HMRC SDLT calculator for stamp duty — Credibrate outputs are sense-checked against those tools at multiple income and property price points before any update is published. Discrepancies of more than £1 in annual figures trigger a review of the underlying calculation.

Update cadence

Tax rates, thresholds, and allowances change on 6 April each year at the start of the new UK tax year. Credibrate updates all affected calculators at the start of each tax year, based on the rates confirmed in the preceding Autumn Budget and Spring Statement. Mortgage rate data is reviewed monthly. Other data — DVLA VED bands, National Living Wage rates, pension annual allowances — is updated as changes are announced. Each calculator page shows the tax year or last-updated date to which its figures apply.

How Credibrate is funded

Some calculator pages contain affiliate links to mortgage brokers and other financial services providers. If you click an affiliate link and go on to use that service, Credibrate may receive a commission. This never affects the results shown by any calculator, the order in which information appears, or the editorial content on the site.

Active affiliate relationships: Habito (mortgage broker), accessed via links on mortgage and property calculator pages. Affiliate commissions are paid by the provider and do not increase the cost to you. This site is a member of the Webgains affiliate network.

Where an affiliate link exists, it is routed via an internal redirect path and the click is logged before redirecting. A disclosure line is included in the footer of every page on which an affiliate link appears.

Editorial independence

Calculator results are determined entirely by the formula and the data entered. Affiliate relationships and commercial arrangements have no influence on how any calculation is performed or what results are shown. A mortgage repayment calculator returns the mathematically correct figure regardless of which lenders are referenced on the site.

Content decisions — which calculators to build, how methodology is explained, which sources are cited — are made independently of any commercial relationship.

Corrections and accuracy

Tax and financial calculations are verified against HMRC and equivalent official sources before publication. If you believe a calculation is incorrect or that a rate or threshold is out of date, please report it via the contact page. Errors are investigated and corrected within five working days. The correction is reflected in the last-updated date shown on the relevant calculator page.

About the author

All calculators and content on this site are built and maintained by Richard Ross. Richard has spent over a decade working in UK financial services, with direct experience across consumer banking, investment products, and personal finance. He built Credibrate to provide free, accurate UK financial calculators without the comparison-site clutter. Read more on the About page.