This is the arithmetic everybody once knew and most of us stopped trusting ourselves on. Nothing about it is hard, and that is exactly why the mistakes are quiet: a percentage taken of the wrong base, or a discount and a tax applied in the wrong order, comes out looking flawless and is still wrong. There is no warning bell, because the answer is a perfectly ordinary number. So these pages show the working rather than only the result — which figure became the hundred percent, what was multiplied by what, and in which order — because that is the part you actually need to check. The significance tests that used to sit here moved to their own category: they are a different job with a different failure mode, and mixing them in made both harder to find.
Tools in this category
Ratio Calculator
Solve a proportion — give three terms of a : b = c : d and get the fourth, the scale factor and the ratio in lowest terms.
Percentage Calculator
Work out a percentage of a number, the result of adding or removing it, and the percentage change between two values — all at once.
Suggest a tool
Tell us what is missing and where its numbers should come from.
What should it convert, calculate or generate, and where would the authoritative numbers come from? A link to the standard or publication that defines them is the most useful thing you can send.
Opens your mail app with this page already filled in.
How these tools are built
Every tool in this category runs entirely in your browser: there is no upload, no account and no request to a server once the page has loaded. The numbers behind them are transcribed from the organisation that defines them — NIST for units, the W3C for web formats, the WHO for health measures — and each page names its source next to the formula. A tool whose reference values stop reproducing fails the build and does not ship, which is the whole reason the catalogue grows slowly.