About us

CalcTools is a small, independent website of free online calculators. We build tools for the questions people actually ask: how much fuel a trip will use, what a body-mass index means, how much downtime a 99.9% uptime promise allows, or how much two-stroke oil goes into a tank of petrol.

How we build a calculator

Every calculator starts with the formula and where it comes from: a health organisation such as the WHO, an engineering standard, a service-level definition or a manufacturer's guideline. We write the calculator, then check it against worked examples by hand before it goes online. The same worked examples appear on the page, so you can check our arithmetic yourself.

Each page explains:

• what the calculator does and what it does not do,
• the formula, in plain words,
• at least one worked example with real numbers,
• common mistakes and limits of the method, and
• the sources we used.

What we stand for

• Show the working. A number without an explanation is hard to trust.
• Free, without sign-up. Every tool works instantly in your browser.
• Your numbers stay with you. Calculations run on your own device. We do not store what you type.
• Honest limits. Health and money calculators give estimates for information only, never a diagnosis or financial advice.

Corrections and suggestions

If you find a mistake, think a formula should be explained better, or would like a calculator we do not have yet, please write to us at [email protected]. We read every message and fix confirmed errors quickly. See also our disclaimer and privacy policy.