What this site is
percentagecalculator.tools is a collection of twelve percentage calculators covering the arithmetic people actually need day to day: working out a discount, adding tax, converting a margin to a markup, splitting a bill, reversing a percentage to find the original number. Each one runs the moment you type. There is no sign-up, no paywall, no app to install and no “calculate” button to hunt for.
Why it exists
Percentages are the piece of school maths that follows people furthest into adult life, and they are also the piece most likely to go quietly wrong. A price that goes up 20% and then down 20% does not return to where it started. A 30% margin and a 30% markup are different numbers. “Increase by 15%” and “15% of” answer different questions. Most calculators will give you a number without ever mentioning that these distinctions exist.
So this site does two jobs. The calculators give you the answer. The guides explain the maths behind it — including the specific mistakes that cost people money — so that next time you might not need the calculator at all. We think that is the right trade, even though it is a slightly strange thing for a calculator site to want.
How it works
Every calculation happens in your browser. Nothing you type is sent to a server, stored, or logged — there is no server-side component that could receive it. That means the tools are fast, they work on a bad connection, and your numbers stay yours. The privacy policy sets out the full picture, including the analytics and advertising cookies that are present.
How the maths is checked
The formula behind each calculator is covered by an automated test suite that runs on every change, including the edge cases that tend to break these things: zero denominators, negative inputs, percentages over 100, and rounding at the second decimal place. Each tool page shows the formula it uses and a worked example, so you can check our arithmetic rather than take it on faith. If you find a case where a calculator is wrong, we would genuinely like to know — the contact form goes straight to us.
Who builds it
The site is built and maintained by The Idea Works, an independent studio that makes small, useful web tools. It is one of several — the others include a colour picker and a set of text tools. They share a philosophy: do one thing, do it fast, explain it properly, and do not make the visitor pay in attention for the privilege.
How it is funded
Advertising, and nothing else. The site is free, there is no premium tier, and we do not sell data — we do not collect any that would be worth selling. Ads pay for the domain, the hosting and the time. We keep them to fixed positions that do not move, expand, or cover what you came to read, and there are none at all on the policy pages or the 404 page.
A note on where you are
This site is written for a global audience. Currency examples are illustrative rather than tied to one country, and anything genuinely region-specific — sales tax rates, tipping conventions — is split into its own clearly labelled guide rather than being presented as the default. If you hit something that assumes the wrong country, tell us and we will fix it.