Every country answers "what do I actually take home?" with a completely different machine. I built four of them: AfterTaxly computes take-home pay for the UK, Australia, New Zealand and Ireland, each from that country's official rates, each checked line by line against the official calculators.
The UK hides a 62% marginal rate that appears on no rate card. Earn between £100,000 and £125,140 and your tax-free personal allowance shrinks by £1 for every £2 over £100,000 — so every extra £100 is taxed at 40%, loses £50 of allowance taxed at 40% again, plus 2% National Insurance. £62 gone from £100, in a system whose headline rates say 40%.
Australia layers a flat 2% Medicare levy, a low-income offset that phases out, and HELP student-debt repayments collected through payroll like a tax — since the 2025-26 reform they are marginal, charged only on the slice above the threshold. Model them the old way, as a percentage of total income, and every payslip with a student debt is wrong.
Ireland stacks income tax with credits instead of allowances, a separate surcharge called USC with its own bands, and PRSI — whose employee rate changes during the tax year, so the engine has to blend a part-year rate change instead of storing one annual number.
New Zealand is the cleanest, except for the ACC levy: an accident-insurance charge priced per $100 of earnings up to a cap, so the ceiling is baked into the rate itself.
One rule held across all four: every figure was verified against the official source before it was coded — and where an official figure did not exist yet, the feature did not ship. An empty slot in the navigation is a better product than a confident wrong number.