Accuracy you can check
HMRC publishes test cases for payroll software every year. Every one of 2026/27’s cases passes, to the penny, on every change: in the engine, and the tax, NI, directors’ NI and student loan deductions through the API as well.
| HMRC’s 2026/27 test data | Version | Cases | Passing |
|---|---|---|---|
| Income tax, rest of UK (cumulative, week 1/month 1, BR, D, K and large codes) | v1.0 | 64 | 64 |
| Income tax, Scotland | v1.1 | 64 | 64 |
| Income tax, Wales | v1.0 | 40 | 40 |
| National Insurance, all 16 category letters, each threshold and the penny either side | v1.0 | 896 | 896 |
| Directors’ NI: four directors’ years, week by week (annual method, pro rata, alternative arrangements) | v1.0 | 4 | 4 |
| Student loans, plans 1, 2, 4, 5 and postgraduate (thresholds and deductions) | v1.0 | 150 | 150 |
The files are HMRC’s: Software developers: payroll test data 2026 to 2027.
Where HMRC’s data doesn’t reach
Pensions, salary sacrifice, childcare vouchers, benefits in kind, the High Income Child Benefit Charge, bonuses, statutory pay and the contractor calculations have no HMRC test files. Each is tested against cases worked by hand from the gov.uk rule it follows, with the workings written beside the test, and take-home has been cross-checked against a well-known salary calculator, every penny of difference explained.
Every response says what produced it
The tax year, the engine’s version and the HMRC test data it passes come back with every calculation, with the assumptions made and the gov.uk sources for the rates.
Use the tested numbers
Every endpoint on every plan.