Methodology and sources

One parameter file

Every rate, band, credit, minimum wage and leave rule lives in lib/manifest.js, grouped by tax year. No formula anywhere on the site contains a hand-typed percentage: they all read from there.

Adding a new year is therefore a matter of copying a block and changing the figures, without touching the calculation engine.

Human verification

Each source carries the date a human checked the figures against it. That date is shown on every calculator and every guide. No legal figure updates itself: a change on the Revenue website does not reach the site until somebody reviews it.

Automated tests

The calculation engine has 105 unit tests, run against the very files the browser loads. They cover exact band boundaries, low and high incomes, shifts that cross midnight, breaks longer than the shift, negative and empty input, years where a rate changes mid-way, and comparisons with hand-computed cases.

What this is not

This is not tax, employment or legal advice for your situation. We cannot see your Revenue record, your contract or your contribution history, and those three things decide your actual pay. Where a result depends on something we cannot check, the result says so.

Corrections

If you find a figure that is wrong, write to diego.bernabeu.martinez@gmail.com with the link and the official source. Corrections to legal figures take priority over everything else.

Sources used