The metric system explained — and why it works
A short tour of the metric (SI) system, its base units, prefixes, and why it has conquered most of the world.
The metric system was the first deliberately decimalised measurement system in modern history. Everything is built around three principles:
1. Powers of ten. Every relationship between units is a power of ten. 1 kilometre = 10³ metres. 1 centimetre = 10⁻² metres. You never have to remember that 1 mile = 5,280 feet = 1,760 yards.
2. Distinct prefixes for distinct scales. kilo, mega, giga, tera for big things. milli, micro, nano, pico for small things. The same prefix works across units (kilogram, kilometre, kilobyte).
3. Universal base units. The 7 base units are defined by physical constants, not artefacts. Since 2019 the kilogram is defined by fixing Planck's constant — there's no prototype in a vault.
The system was developed during the French Revolution (1795) and refined through the 19th century. SI (Système International d'Unités) was formalised in 1960 by the Bureau International des Poids et Mesures.
Why the US hasn't converted
Three countries still use non-metric measurement in everyday life: the United States, Liberia, and Myanmar. The UK uses a hybrid (miles for road signs, kilograms for vegetables). The cost of converting every road sign in a country of 4 million miles of roads is non-trivial — the US has debated it several times and the legislation has never passed.
What this site uses
We organise units by category. Each category has an SI base: metre (length), kilogram (mass), kelvin (temperature), etc. Most conversions factor through this base. Temperature is the exception: it's affine, meaning there's an offset on top of a scale change.