Tools that show their work

A number with no working is hard to trust. You cannot see if it was rounded, or how long the tool thinks a month is.

So each calculator and converter here shows the rule it uses and an example with real numbers. If your result differs from ours, compare your steps with the example to find the difference.

Exact factors

An inch is exactly 2.54 centimetres. That has been its definition since 1959. Each converter uses exact factors like this one and converts through one base unit, so rounding errors do not build up.

Money to the penny

The loan calculator counts money in whole pence or cents. It rounds the interest once each period, and the last payment takes up what is left. The balance ends at exactly zero.

Dates by the calendar

The age calculator counts whole years and months first, then the days left over. When a month is too short, it uses the last day of that month. So someone born on 29 February has a birthday on 28 February in other years.

Tests next to the code

The calculators and converters keep their maths in plain functions, with tests next to them. The tests check known answers and hard cases, like a leap year or a loan at 0%.

Every tool runs in your browser. When you open a file, your device does the work, and the tool never uploads it.

Some tools, like the video tools, download extra code the first time you open them. That code also runs on your device.

Every tool is free, with no account and no daily limit. The site counts page views with Cloudflare Web Analytics, which does not use cookies.

11 of them carry a Preview label. They work, but no one has checked them by hand yet.

One person, Tushar, builds and runs zerouploads. If a result looks wrong, email hello@zerouploads.com with the numbers you used.