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.
Two tiers of calculator
We run two kinds of tool. The multi-country calculators (income tax, capital gains, stamp duty and so on) cover several countries at a useful but deliberately simpler level, so they can stay comparable across borders. The deep, country-specific calculators go much further for a single jurisdiction, modelling the factors that genuinely change the answer.
Deep UK calculators — what they model
Our United Kingdom suite is built factor-by-factor for the 2026/27 tax year (financial year 2026 for corporation tax):
- Income tax & take-home — rest-of-UK and separate Scottish bands, Class 1 National Insurance, the three pension methods (salary sacrifice, net pay and relief at source, which differ on NI), student loan Plans 1/2/4/5 and postgraduate, the £100,000 personal-allowance taper, benefits in kind, Gift Aid, Marriage Allowance and Blind Person's Allowance.
- Property CGT — full HMRC computation with buying/selling costs, improvements, Private Residence Relief by ownership period, joint ownership, capital losses and the £3,000 annual exempt amount.
- Self-employed — profit build-up, income tax (incl. Scotland), Class 4 and Class 2 National Insurance, the £1,000 trading allowance, pension relief, student loans and payments on account.
- Inheritance tax — estate build-up, the nil-rate band and transferable bands, the residence nil-rate band with its £2m taper, spouse and charity exemptions, the reduced 36% charity rate and 7-year gift taper relief.
- Stamp duty — all three regimes (SDLT, LBTT, LTT) with first-time-buyer relief and the additional-property and non-resident surcharges, band by band.
- Corporation tax — profit build-up, the 19% and 25% rates with marginal relief, and adjustments for associated companies and short accounting periods.
What we deliberately leave out
The multi-country tools leave out personal circumstances (pension contributions, salary sacrifice, student loans, tax codes, household status). The deep UK tools model most of these, but every tool still leaves out:
- State, regional, provincial and municipal taxes (e.g. US state income tax, Spanish regional rates, German church tax).
- Less common reliefs and special asset, property or business classes beyond those each tool names — for example lettings relief, Business Asset Disposal Relief, trusts, agricultural and business property relief, R&D and group relief, and leasehold or mixed-use property.
- Directors' annualised NI, multiple jobs, K codes, basis-period adjustments and instalment-payment timing.
- 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.