The 7 SI base units — what they are and how they are defined

The modern metric system has seven base units. Every other unit you encounter is either one of these (with a prefix) or derived from them.

The seven

| Quantity | Unit | Symbol | Defined by (since 2019) | |---|---|---|---| | Length | metre | m | speed of light in vacuum | | Mass | kilogram | kg | Planck constant | | Time | second | s | cesium-133 hyperfine transition | | Electric current | ampere | A | elementary charge | | Temperature | kelvin | K | Boltzmann constant | | Amount of substance | mole | mol | Avogadro constant | | Luminous intensity | candela | cd | luminous efficacy of 540 THz radiation |

Why the change in 2019

Before 2019, the kilogram was defined as the mass of the International Prototype Kilogram — a platinum-iridium cylinder stored in a vault in Sèvres, France. Every other country's kilogram had to be measured against it. The cylinder was slowly losing mass relative to its copies, which is a fun but unsustainable definition.

After 2019, the kilogram is defined by fixing Planck's constant h = 6.62607015 × 10⁻³⁴ kg·m²/s. This means the definition is reproducible in any well-equipped metrology lab, anywhere, forever.

Derived units you'll use

Almost all the units on this site are derived:

- 1 newton = 1 kg·m/s² - 1 joule = 1 kg·m²/s² - 1 watt = 1 kg·m²/s³ - 1 pascal = 1 kg/(m·s²) - 1 hertz = 1/s

Practical implication

If you ever argue with someone about whether grams or kilograms are the "real" unit: the kilogram is base. The gram is a prefix-altered version (1 g = 10⁻³ kg).

The site uses the correct base in every category, so 1 m = 1, 1 kg = 1, 1 s = 1. Conversions away from the base are multiplicative factors.