22 categories, 181 units, converted in your browser
Every unit you actually need, on definitions rather than rounded factors
One row does the whole job: a number, a unit, a result. Twenty-two categories sit above it and everything else is a panel you open when you want it, so the tool is usable before you have read anything on the page.
The numbers come from definitions, not from a table someone typed. A pound is 0.45359237 kg exactly, an inch is 2.54 cm exactly, a mile is 1609.344 m, a US gallon is 231 cubic inches. Those are the constants the units are derived from, so the conversion is exact and only the display is rounded, to as many decimals as you ask for.
Three kinds of unit need three kinds of arithmetic, and the tool keeps them apart. Most are a single factor. Temperature is affine, with an offset as well as a scale, which is why 0 C is 32 F and not 0 F, and why anything below absolute zero is refused instead of converted. Fuel economy and running pace are reciprocal: more litres per 100 km is fewer miles per gallon, so they are inverted rather than multiplied.
Decimal and binary prefixes are separate units. 1 kB is 1000 bytes and 1 KiB is 1024, so a 500 GB drive reads as roughly 465 GiB, and the tool says which one it means instead of picking for you.
Everything happens in this tab. There is no API call behind the page and nothing you type is sent anywhere; the saved pairs and the recent conversions are kept in this browser and nowhere else.
The most important capabilities, explained in practical terms.
Length, mass, temperature, area, volume, speed, pace, time, pressure, energy, power, data, data rate, fuel economy, angle, frequency, force, torque, density, acceleration, cooking and typography. 181 units in all, and switching category never leaves the page.
One panel restates what you typed in every unit of the category at once, each row clickable to make it the new target. It is faster than converting five times and it catches the unit you did not think to ask for.
The pair on screen comes with its own line of arithmetic, so you can see that inches are centimetres times 0.393701 rather than take the number on faith. Affine and reciprocal pairs show their real form, not a fake multiplication.
Type psi, knot, megabyte or stone and get the unit with its category, wherever it lives. Aliases are matched too, so foot, feet and ft all find the same unit.
Celsius, Fahrenheit, Kelvin and Rankine convert through kelvin with the offset applied, which is the one conversion a scale factor alone gets wrong. Anything below absolute zero is refused with a reason rather than answered.
kB, MB, GB and TB are powers of ten; KiB, MiB, GiB and TiB are powers of two. Both are on the page as separate units, with bits and transfer rates in their own category, so nothing is silently off by 7%.
A clear path from the official source to your first successful workflow.
Use one of the official links available for your device or browser.
Follow the store instructions or open the web application. No third-party installers.
Review the options, choose your preferences and start with the core workflow.
A transparent look at the core technologies used to build and maintain this product.
Your data stays on your device. Always.
