Accuracy tests
Every time this site is built, the calculation engine is run against the cases below and the build fails if any of them is off by more than a cent. A larger suite (30 tests) runs in the engine's own test runner before any rate file is changed. Last build: 25 October 2026. Engine and rate tables last reviewed: 25 October 2026.
| Case | Expected | Engine | Result |
|---|---|---|---|
| US: $75,000 single, Texas gives $61,592.50 take-home (2026 brackets, std deduction $16,100, FICA) | 61,592.50 | 61,592.50 | pass |
| US: $60,000 single, federal only gives federal tax $5,020 | 5,020.00 | 5,020.00 | pass |
| US: $100,000 married filing jointly gives federal tax $7,640 | 7,640.00 | 7,640.00 | pass |
| US: $250,000 single gives Social Security capped at $11,439 (wage base $184,500) | 11,439.00 | 11,439.00 | pass |
| US: Illinois $60,000 gives state tax 4.95% of ($60,000 minus $2,925) = $2,825.21 | 2,825.21 | 2,825.21 | pass |
| US: California $75,000 single gives state tax $2,787.85 and SDI $975 | 2,787.85 | 2,787.85 | pass |
| UK: £30,000 England 1257L gives £25,119.60 | 25,119.60 | 25,119.60 | pass |
| UK: £150,000 England gives income tax £53,703, NI £5,010.60 | 53,703.00 | 53,703.00 | pass |
| UK: £50,000 Scotland gives income tax £8,982.05 across four bands | 8,982.05 | 8,982.05 | pass |
| UK: Plan 2 loan at £35,000 gives £505.35 | 505.35 | 505.35 | pass |
| VAT: remove 20% from £100 gives £83.33 net | 83.33 | 83.33 | pass |
| VAT: add 19% to €100 gives €119 | 119.00 | 119.00 | pass |
What the tests cover
- US federal brackets and standard deductions for 2026 from Rev. Proc. 2025-32, for single, married filing jointly and head of household. The Social Security wage base. Additional Medicare tax.
- State engines for the flat tax, bracket and no tax cases, plus California SDI and the New York City resident tax.
- UK 2026/27 income tax (England, Wales, Northern Ireland and Scotland), National Insurance, student loan plans, pension methods, personal allowance taper and tax codes.
- VAT add, remove and round-trip arithmetic for all 31 countries in the rate file.
Known limits (and what is coming)
- US federal withholding uses the annual-liability method. The per paycheck figure your employer calculates with Publication 15-T can differ by a few dollars. Version 0.2 adds the 15-T tables.
- New York's tax benefit recapture above $107,650 and the Illinois exemption phase out are not yet modelled.
- UK figures are annual. In-year cumulative PAYE and emergency codes can differ month to month.
Found a discrepancy? Tell us with the inputs and the source you compared against. Confirmed corrections go into the changelog.