How these numbers are kept right

A rate on this site is read off its source page by a person, on a recorded date, against the exact sentence that states it — and that check expires on its own. Where a figure has not been through that yet, the calculator using it says so, in amber, where you read the answer.

A calculator is only as good as the numbers inside it, and tax figures go stale every April. Most sites deal with that by writing the rates into the code and hoping someone remembers. This one is built the other way round, and the whole method is set out below so you can judge it rather than trust it.

Where the numbers come from

No rate is written into the code of this site. Every one lives in a dated file alongside the page it came from, the dates it applies between, and the sentence on that page that states it. Changing a figure is therefore a change to data with a source and a date attached, never a quiet edit to a formula.

Most sources are gov.uk or HMRC, because most of these are taxes. Some are not: NHS Agenda for Change pay scales are published by NHS Employers and pension contribution tiers by the NHS Business Services Authority, which are the bodies that set them. The rule is the same either way — cite the body that decides the figure, and quote the sentence where it says so.

Every figure and its source is published in full, so you can follow any number back to the page it was taken from.

How each one is checked

A tool fetches each cited page and prints the sentence containing the value next to what this site claims. A person reads the two and signs the entry off. What gets recorded is the date and the sentence they read – not merely that someone once looked.

Doing this the first time found that the numbers were right and the citations were not: several pointed at a page that described a scheme without ever stating a rate. That is the kind of error a spot-check by eye never catches, and it is why the check works from the sentence rather than from the page.

And then a machine watches it

Most of these figures sit in a table on their source page. Where one does, the check records exactly which cell it came from — the section, the row and the column, in the source’s own words.

This paragraph used to say how many carried an address like that. It was written once and never updated, which is the same failure as a hand-typed “last checked” date — so the number is gone and the sources page, which is generated from the data every time the site is built, is where to count them.

After that a machine can re-read the same cell and confirm the number has not moved, which it does far better than a person re-reading a page for the fourth time. What it cannot do is decide which cell to read. Both “20%” and “1.85%” appear in the National Insurance table; which one applies to an ordinary employee is a judgement, and it was made once, by a person, and written down where you can check it.

So the division is: a person decides what a figure means, once. A machine confirms it has not changed, continually. Anything the machine cannot resolve — a value that has moved, a table that has been restructured, a row that no longer exists — stops being treated as checked and waits for a person.

What “not yet checked” looks like

A figure that nobody has read against its source yet is not treated as if somebody had. The calculator carries an amber “Rates not yet checked against their source” line instead of a date, and that badge is scoped to the figures that page actually uses — so one unchecked group ambers the calculator that relies on it and leaves the rest of the site reporting its real last-checked date.

It is deliberately not possible for a new figure to inherit somebody else’s tick. Adding one unchecked rate to a file marks that file unchecked until the new rate has been read, which is why the state is worth believing.

How a check expires by itself

The sentence that was read is stored as a fingerprint. When the source rewrites that wording – which is what usually happens when a figure changes – the fingerprint stops matching and the entry stops counting as checked. Nothing has to be noticed for that to happen.

A tax year has an end date too, and every figure records the dates it applies between. When the year turns, the site says so on the page rather than carrying on quietly with last year's numbers. That is the failure this is most guarded against: figures that are wrong and look completely normal.

What the date on each page means

The line at the foot of every calculator reads Rates last checked and a date. That date is the oldest check in the file, not the newest – so it is the weakest claim the data can support, rather than the most flattering one. If a single figure has not been re-read since August, August is the date you see.

If any figure in a file has not been checked at all, no date is shown. The page says the rates have not been checked instead.

The thirteen figures that live in prose rather than in a table can only be re-checked by a person, so in practice they are usually the ones holding that date back. That is the right way round: the date should be dragged down by whatever has been looked at least recently, not lifted by whatever has been looked at most.

How the sums are done

Money is worked out in whole pence as integers, never in decimal fractions of a pound, because a tax tool that is a hundredth out is a tax tool nobody should trust twice. Each calculator is covered by tests that check its answers against figures worked through by hand, including the awkward cases – the taper that creates a 60% band, the round-down that applies to student loan repayments every month, and National Insurance being charged per pay period rather than per year.

All of it runs in your browser. The figures you type are never sent anywhere, which is also why there is no account and no sign-in. What little is collected is set out separately.

What this site is not

FullWorkings is an independent publisher of calculators. It is not a firm of accountants or financial advisers, it is not regulated by the Financial Conduct Authority, and nothing here is personal advice. The figures are estimates to help you understand a rule or check a payslip – not a substitute for advice about your own circumstances, and not something to file a return from without checking.

Every calculator also names what it does not cover, on its own page, rather than leaving you to discover the gap. Those limits are part of the answer.

If you find a mistake

Tell us and it gets fixed. Corrections are the most useful thing anyone can send: a figure, the page you found it on, and what you believe it should be. Anything that turns out to be wrong is corrected and its source re-checked.

corrections@fullworkings.co.uk

That address exists for exactly this. It is not a support line and there is nothing to sign up to.

Last updated 31 August 2026 Take-home pay calculator