Trust and accuracy
Calculator methodology
Every calculator has a written specification that identifies its inputs, formula, period, exclusions and primary source before the interface is released.
Last reviewed: 13 September 2026
Source hierarchy
We prefer tax authorities, government departments, regulators and legislation. A secondary source may be used to explain context, but it does not replace the primary source for rates or thresholds.
Calculation testing
We test zero values, values around every threshold, representative ordinary cases and high values. Results are checked with manual calculations and, where appropriate, an established independent calculator. A disagreement is investigated rather than averaged away.
Dates and updates
Country rules are versioned by income year or effective date. Pages show the period they use and a review date. A rate change triggers a new test pass and a content review.
Known limitations
A calculator cannot capture every personal fact. Each tool states what it leaves out, such as tax credits, exemptions, employer arrangements, lender fees or irregular payroll rounding.