The metric system is the most widely used measurement system in the world, and its modern form — the International System of Units (SI) — underpins nearly all science and trade. Its great advantage is consistency: everything scales by powers of ten.
The SI base units
SI defines seven base units, from which all others are derived. The ones you’ll meet most often are:
- Metre (m) — length
- Kilogram (kg) — mass
- Second (s) — time
- Ampere (A) — electric current
- Kelvin (K) — temperature
Derived units combine these: speed is metres per second, force (the newton) is kg·m/s², and pressure (the pascal) is newtons per square metre.
Metric prefixes
The real power of the metric system is its prefixes. Attach one to a base unit and you scale it by a power of ten:
| Prefix | Symbol | Factor |
|---|---|---|
| kilo | k | ×1,000 |
| hecto | h | ×100 |
| deca | da | ×10 |
| deci | d | ÷10 |
| centi | c | ÷100 |
| milli | m | ÷1,000 |
| micro | µ | ÷1,000,000 |
So a kilometre is 1,000 metres and a millimetre is one-thousandth of a metre. The same prefixes apply to grams, litres, watts, amps, and more — which is why once you learn them, most of the metric system falls into place.
Converting within metric
Because every step is a power of ten, converting within the metric system just moves the decimal point. Converting out of metric — to inches, pounds, or Fahrenheit — is where a tool helps. Start at the all converters index or jump to length, weight, or temperature.
Metric and everyday life
Even in countries that use imperial units day to day, science, medicine, and manufacturing run on metric. Learning to think in metric — and to convert quickly when you need to — is one of the most broadly useful numeric skills you can have.