How we calculate

Every calculator on FairNetPay is built from the tax authority's own publications — payroll formulas, withholding tables, rate notices and laws — never from another calculator. This page explains how a rate gets from an official document into a result, how we test it, and what the calculators deliberately leave out.

1. Official sources only

For each country and tax year we read the primary source: for example HMRC's rates and thresholds for employers, the ATO's Schedule 1 coefficients, the CRA's T4127 payroll formulas, the IRS's Publication 15-T and each US state's own withholding guide, Germany's Programmablaufplan (PAP) from the Federal Ministry of Finance, or Japan's 源泉徴収税額表. Every figure is written into a rules file together with the document it came from. The sources are linked at the bottom of every calculator.

Where a value could only be found in a secondary source (a payroll provider, the press, or an announcement not yet in law), we say so: it is listed as provisional in our internal re-verification list and replaced with the official figure when it is published.

2. The tax authority's own method, not an approximation

Payroll withholding is often not "your annual tax divided by 12". Many authorities publish an exact procedure — annualise the pay, deduct allowances in a fixed order, apply a table, round in a specific way, then divide. We implement that procedure as published, including its rounding rules, so that the per-paycheck figure matches what payroll software certified for that country would withhold. Where a country only publishes annual rates, we say that the result is an annual estimate.

3. Tested against official worked examples

When a tax authority publishes a worked example — "a weekly wage of $500, single, three allowances, withholding $9.58" — we reproduce it in an automated test, to the cent. We add tests at every threshold, at zero income and at very high incomes, and for each option a calculator offers. The whole test suite runs before every release; a change that breaks any test is not published. Where an official example contradicts its own tables, we do not guess: the calculator is held back until the authority corrects it, and the issue is noted.

4. Dated, so you know what you are looking at

Each calculator shows the tax year it applies and the month its rules were last checked. When rates change during a year (some US states change tables mid-year; Turkey revises its severance cap every six months) the calculator states which version it uses and from which date.

5. What the calculators leave out

• Personal circumstances that only your employer or tax office knows: benefits in kind, salary advances, garnishments, back pay and prior-year adjustments.
• Local taxes where they are set city by city (for example most US local income taxes), unless the page says otherwise.
• Cumulative payroll effects: a calculator shows a normal pay period, not the catch-up that happens when you start mid-year or change tax code.

Each calculator lists its own assumptions in the notes under the result.

6. How rules are kept current

We keep an update calendar of every date on which rates change in each country — 1 January in most of Europe and North America, 6 April in the UK, 1 July in Australia, Budget days in India, South Africa and Pakistan — and re-check the official sources around each date. Changes are recorded in the changelog; corrections of published figures are listed on the corrections page.

Verification status by country

Generated from the pages themselves at build time.

Found a difference?

If your payslip or the tax authority's calculator gives a different figure, please tell us with the inputs you used. See the editorial policy for how we handle reports.