Enter a value and pick your units to convert — conversions run locally in your browser, with nothing uploaded.
How it works
- Most categories convert through a fixed multiplier against an SI base unit (metres, kilograms, seconds, pascals, joules, watts…), so results are exact to full floating-point precision.
- Temperature and fuel economy are non-linear, so they use the correct formulas (Celsius/Fahrenheit/Kelvin, and reciprocal MPG↔L/100km) rather than a straight multiplier.
- All conversions happen instantly in your browser as you type.
Convert between metric and imperial units across 17 categories — length, weight, temperature, area, volume, time, speed, angle, force, pressure, energy, power, frequency, digital storage, data rate, fuel economy and cooking measures — instantly and entirely in your browser. Nothing is uploaded, so it works offline once loaded.
Frequently asked questions
Are the conversions exact?
Length, weight, area, volume and data conversions use exact, internationally-defined multipliers against an SI base unit, so results are precise to full floating-point accuracy. Temperature uses the exact Celsius/Fahrenheit/Kelvin formulas rather than a linear approximation.
Why is 1 KiB shown as 1.024 KB?
Because KB (kilobyte) is 1,000 bytes while KiB (kibibyte) is 1,024 bytes. The converter keeps the decimal (KB, MB, GB) and binary (KiB, MiB, GiB) families separate so you always know which standard you're using.
Does it handle fuel economy both ways?
Yes — it converts between miles-per-gallon (US and UK) and litres-per-100km, which are inverse scales, so the maths is done correctly rather than treated as a straight multiplier.
Is my data sent anywhere?
No. Every calculation runs in JavaScript on your device. There is no server call, so you can use it with sensitive numbers and even offline after the first visit.
Related guide: Convert units without silent errors →
Measurement systems
Most conversions move between two families of units: the metric (SI) system used almost everywhere, and the imperial or US customary units still common in a few countries. The tool handles the arithmetic; understanding which system you're starting from prevents the classic mistakes.
SI prefixes
Metric prefixes are powers of ten, which makes them easy to chain. Milli- means one-thousandth, centi- one-hundredth, and kilo- one thousand. So 1 kilometre is 1,000 metres, and 1 centimetre is 0.01 metres. Because each step is a power of ten, converting within metric is just moving the decimal point.
A useful relationship
One inch is defined as exactly 2.54 centimetres, which anchors most length conversions between the two systems. From there, 1 foot (12 inches) is 30.48 cm, and 1 mile is about 1.609 kilometres.
Precision and rounding
Converted values are often irrational or long decimals, so the result is rounded for readability. For everyday use that's fine; for engineering or dosing, keep more decimal places and round only at the very end, so rounding errors don't accumulate across several steps.
Why 'lb' means pound
The English word pound traces through the Latin expression libra pondo. The historical libra is why the abbreviation is lb. The international avoirdupois pound is now defined as exactly 0.45359237 kilograms.
The metre stopped being a metal bar
The metre was once represented by a physical prototype. Its definition later moved through atomic radiation and, in 1983, to the distance light travels in vacuum during a specified fraction of a second. Measurement shifted from artifacts to reproducible physical constants.
Even the kilogram left its metal cylinder behind
The kilogram was historically tied to a physical prototype. In the 2018 SI revision its definition was tied to a fixed value of the Planck constant, improving long-term reproducibility.