Our Methodology & Editorial Standards

We build free calculators for US and UK users. Because many of them touch money, tax, and health, we are deliberate about the formulas we use, the sources we rely on, and the limits of every estimate. This page explains how our tools work and how we keep them accurate.

How our calculators are built

Every calculator implements a published, widely-accepted formula — not a black box. For example, our mortgage calculator uses the standard amortization formula, our BMI calculator uses the WHO/CDC body-mass-index formula (weight ÷ height²), and our compound interest calculator uses A = P(1 + r/n)^(nt). Where a calculation has more than one accepted method, we say which one we use and let you switch where it matters.

Sources we rely on

For country-specific figures we use official primary sources and update them when the rules change:

• US tax & payroll: the Internal Revenue Service (IRS) for federal income-tax brackets, the standard deduction, FICA rates, and contribution limits (e.g. the 2026 401(k) limit).
• UK tax & payroll: HM Revenue & Customs (HMRC) for Income Tax bands, the Personal Allowance, National Insurance, VAT, and student-loan thresholds.
• Interest & inflation: the Federal Reserve and the U.S. Bureau of Labor Statistics (CPI) for the US, and the Bank of England and the Office for National Statistics (CPIH/RPI) for the UK.
• Health: the CDC, NIH/NHLBI, and the NHS for BMI categories, calorie and body-composition formulas.
• Property & sales tax: the Tax Foundation and official state revenue departments for US sales-tax and property-tax rates.

How often we update

Evergreen tools (geometry, unit conversion, date and time math) do not change and carry no year. Rate-driven tools (income tax, VAT, sales tax, 401(k), inflation) are reviewed at the start of each tax year and whenever a rate is changed by the IRS, HMRC, or another authority. Each rate-sensitive page shows a "last updated" signal so you can see how current it is.

US and UK localisation

Several calculators have dedicated United States and United Kingdom versions because the underlying rules genuinely differ — US mortgages use PMI, FHA/VA loans and property tax, while UK mortgages use deposits, loan-to-value and stamp duty; US income tax is federal-plus-state with FICA, while UK income tax is PAYE with National Insurance. We localise currency, terminology, examples and sources for each market, and never present US rules as UK rules or vice versa.

Estimates only — not professional advice

Our calculators give estimates for general information. They cannot account for every personal circumstance, allowance, credit, or local rule. They are not financial, tax, legal, or medical advice. For decisions that matter, please confirm the figures with a qualified professional — an accountant or tax adviser, a mortgage broker or lender, or a doctor — and with the relevant official source (IRS, HMRC, your lender, or your clinician).

Corrections

If you spot a figure that looks wrong or out of date, please tell us via our contact page and we will review it. Accuracy matters to us, and reader reports are one of the ways we keep these tools reliable. You can also read our editorial policy and disclaimer.