How our calculators work
Every Understand Your Tax tool runs on the same transparent, data-driven engine. This page explains the approach so you can judge the results for yourself.
Progressive band calculation
Most income and capital taxes are "slice" taxes: each portion of your income or gain is taxed at its own rate. We apply each band's rate only to the slice that falls within it, then add the slices together — exactly as the tax authorities do. This is why your effective rate (total tax ÷ income) is always lower than your top marginal rate.
Allowances, deductions and credits
Where a country gives a tax-free amount — a personal allowance, standard deduction or annual exemption — we subtract it before applying the bands. Where a country uses tax credits instead (which reduce the tax due rather than the taxable amount), we subtract those after the bands. The UK personal allowance taper above £100,000 is modelled, which is why the calculator can show the effective 60% marginal band.
Social contributions
Income tax alone isn't your full deduction. We model UK National Insurance and US FICA (Social Security + Medicare) directly. For other countries, social contributions can be substantial and vary by scheme; where we don't yet model them, we say so clearly rather than understating your deductions.
What we deliberately leave out
- State, regional, provincial and municipal taxes (e.g. US state income tax, Spanish regional rates, German church tax).
- Personal circumstances: pension contributions, salary sacrifice, student loans, tax codes, marital/household status and dependants.
- Reliefs, exemptions and special asset or property classes beyond the common cases shown.
- Holding-period rules that change whether a gain is short- or long-term.
These omissions keep the tools fast and easy to understand, but they mean every result is an estimate. For a figure you can rely on, confirm with the official authority or a qualified adviser.
Keeping figures current
All rates live in a single dataset per tool, sourced from official government publications and reviewed each tax year. The "last reviewed" date on each calculator tells you when its figures were last checked.