Anyone can put a formula on a web page. Here’s what we do to make our tools useful and dependable, and where their limits are.
Start from a published method
Our calculators use standard, published formulas. Where a tool relies on published guidance, its Sources section links to it. For example, our BMI calculator cites the CDC and the World Health Organization, and our due date calculator cites the American College of Obstetricians and Gynecologists.
Check it before it goes live
We test each calculator against worked examples, and try the kinds of input people get wrong, such as empty fields and impossible dates. File tools are tested with different files, including damaged and password-protected ones.
Keep it simple
Each tool asks only for what it needs, uses US units by default where units apply, and explains its result in plain English. We build for phones first, with the tool at the top of the page.
Keep your data on your device
Our calculators, image tools and PDF tools do their work in your browser. Your files aren’t uploaded to our servers. If a future tool needs our server, its page will say so. Our privacy policy has the details.
Expert review
When a named expert has reviewed a tool, its page shows who reviewed it and when. Please treat every result as an estimate.
Understand the limits
A calculator can only work from what you enter and the method it uses. It doesn’t know your full situation. Results aren’t medical, financial or legal advice.
Tell us if something’s wrong
If you find a mistake, please contact us with the tool’s name and what you entered. We’ll look into it and correct the tool if needed.