Methodology
How every number on WagePrism is produced, and how we keep it right.
Rules as data
Each country and tax year has one rules file: brackets, thresholds, rates and caps, each with the official page it came from and the date we checked it digit by digit. Nothing on the site types a rate by hand; explainers, calculators and salary pages all read from that file.
One engine everywhere
The same calculation code runs when the site is built (for the figures on salary pages) and in your browser (for the calculator), so a salary page and the calculator can never disagree.
Tested against official calculators
Before each release the engine must reproduce cases captured from the government's own calculators. The results are published on the accuracy page.
Two kinds of answer
Payroll takes tax from each pay using a formula; your real tax for the year is settled separately. They usually differ a little. Each calculator says which it shows, and the Australian one shows both.