Where every figure comes from

Sources are primary only: statutes and revenue procedures for the US, gov.uk for the UK, canada.ca for Canada, the ATO for Australia. No figure on this site is sourced from another calculator site, a blog, or a language model.

Every dated constant in the tax-data module carries four things: the figures themselves, a link to the primary source that states them, the date a person last verified them against that source, and the window in which they are effective. None of those fields is optional.

• Omission fails the build. An automated test reads the data module, extracts every exported dated constant, and fails if one is not registered in this inventory. A monitor you have to remember to register with is a monitor that quietly stops covering things.
• Review clocks are enforced. Figures whose error would change what someone pays or withholds are re-checked at least every 120 days; the rest at least every 200. A daily job also re-checks that each cited page is still reachable and still states the key figures we rely on.
• Year-ends are anticipated. 75 days before a block’s effective period ends, it is flagged for succession — which is what catches “the new tax year starts in six weeks and next year’s figures are not sourced yet”, the failure a pure last-checked date misses entirely.
• Prior years are kept, labelled. Superseded blocks are retained so past-year calculations still work, and are marked historical above rather than deleted or passed off as current.

One thing the system deliberately does not do: rewrite figures automatically. A scraper that starts matching the wrong table publishes wrong brackets with total confidence, and authorities publish draft figures that are amended or never enacted. So automation does the tedious half — noticing a figure moved, a year ended, a source vanished — and a person does the half that carries the liability. Being a day late is a non-event; being confidently wrong means someone under-withholds all year.

Anyone — human or machine — can produce a tax figure. What cannot be generated is accountability: where the number came from, when it was last checked, and what happens when it goes stale. An answer from a chatbot carries none of that, and most calculator sites carry little more; the figures are somewhere in the code, of some vintage, checked at some point by someone.

We think the audit trail is the actual product and the calculators are the interface to it, so the trail is public. If a figure above looks wrong, or you know a source has changed before our monitor caught it, report it — a correction with a citation beats a hundred page views. The methodology behind the calculations themselves is on how we calculate.