How we check our tools

A calculator is only worth using if its answers are right. This page says exactly what stands between a formula and the number you read.

One engine, checked against frozen answers

Every tool's arithmetic lives in one place, and the same code runs twice: once when the page is built, to write the worked example and the figures in the text, and again in your browser, to answer as you type. Because it is the same code, the page and the calculator cannot disagree.

That engine is held to a set of frozen test cases. Each case lists the inputs and the answer agreed for them, and the build stops if any case comes out differently. A test's expected answer is never edited to make a test pass.

Numbers in the words come from the engine, not from a writer

No figure is typed into the text of a page. Every number in a sentence is a placeholder filled from the engine's output for a named test case when the page is built. The build refuses a page whose text contains a hand-typed figure, so a stale number cannot sit in a paragraph while the calculator above it says something else.

Rates and slabs live in data files, with their source

Interest rates, tax slabs, caps, thresholds and conversion factors are not written into the code. They sit in data files, each one carrying the name of the source it came from and the date it was taken. Every page that uses one shows that source and that date, so you can see how current it is and check it yourself.

A person checks the words

Finance, tax, scheme and health pages are the ones where a mistake costs the most, so each is read by a person against a bank, regulator or medical reference before it is published, and the page says who checked it and when. A page whose words have not yet been checked is marked as a draft and is kept out of search.

What this does not promise

These tools give estimates. A lender, an employer or a tax office works from your full circumstances and from figures that may have changed since a page was reviewed. Use a result here to prepare and to sanity-check, and use the official figure to act on.

Found something wrong? The contact page is the fastest way to tell us.