Methodology

The source hierarchy

When two sources disagree about a rate or a threshold, the one higher in this list wins. The order is not a preference; it is the order in which the sources are actually authoritative.

1. Legislation and the tax authority's own rate tables. The statute, statutory instrument, budget measure, rate resolution or revenue-procedure notice that sets the number, or the authority's published table of it. This is the only kind of source a figure is ever taken from.
2. Official guidance. The authority's explanatory pages, help sheets and manuals. Used to establish how a rule is meant to be applied — which order operations run in, what counts as included income, how a taper interacts with a cap.
3. Official worked examples. The authority's own published example calculations. Used to verify an implementation end to end: if our figure and theirs differ on the same inputs, our implementation is wrong until proven otherwise.
4. Secondary sources, for cross-checks only. Professional-body commentary and accounting firm summaries can flag that something has changed or that a reading is unusual. They are never the source of a published figure, and they are never cited as authority for one.

Two things follow from this that are worth stating plainly. First, a figure that already sits in a site's configuration is not evidence for itself — configuration is a cache of the official source and it can go stale, so it gets re-verified against the source rather than copied forward. Second, when an official page cannot be retrieved, the correct response is to keep trying until it can be, not to substitute a lesser source.

How rules are versioned

Each site keeps its tax rules in one place, keyed by tax year, and every calculator reads them from there. A rate change is therefore a change to one dataset rather than an edit hunted through calculator code, article text and page copy — the pattern that produces sites where the calculator is right and the paragraph beside it is a year out of date.

• Historical years are retained. When a new tax year starts, the previous year's rules stay in the data and stay reachable. Nothing is silently overwritten.
• Calculators whose answer depends on the year let you choose the year. The selection flows all the way into the calculation, so the historical answer is computed with the historical rules rather than approximated with current ones.
• Pages about a specific year stay pinned to it. A page written about a named tax year keeps computing that year after the year has turned over, because that is what it is for. It does not quietly become a page about a different year.
• Each country keeps its own calendar. The tax year is 1 July to 30 June in Australia, 6 April to 5 April in the United Kingdom, 1 April to 31 March in New Zealand and the calendar year in Canada and the United States. Rules are versioned on the country's own calendar, not on a shared one.

The update process

Rate changes arrive on a known schedule — budgets, autumn statements, indexation announcements, annual inflation adjustments — and occasionally out of schedule when legislation passes mid-year. Either way the sequence is the same:

1. The change is read from the official announcement, not from reporting about it.
2. The new values are added to the year-keyed data, alongside the previous year rather than on top of it.
3. Tests are updated or added so the new value is asserted, and the existing tests for prior years must still pass — a change that alters a historical answer is a bug.
4. The written pages are swept for the superseded figure. Prose drifts from configuration far more readily than code does, so this step is explicit rather than assumed.
5. Nothing is published until the build passes, which includes checks for broken internal links, unreachable pages and pages missing from the site's own index.

Pages that carry substantive tax figures show when they were last reviewed and which official documents they rest on, so the reader can see the age of what they are reading rather than having to assume it.

Authorities by country

Each site implements the rules published by its own country's tax authority, and by the state, provincial or devolved bodies that set rates alongside it.

What the methodology does not cover

A calculator models the general case of a rule. It does not know your residency history, your spouse's income, a private ruling you hold, a treaty position, or the dozen circumstances that make real tax returns individual. Where a rule branches on something the calculator cannot see, it takes the ordinary case and says so. That is a modelling choice, not advice, and the further your circumstances are from the ordinary case the less the output is worth.

See also the standards these methods implement and the testing practice that checks them.