What is a unit? A practical guide
Why every measurement needs a unit, the difference between base and derived units, and why SI exists.
A measurement without a unit is meaningless. "This room is 5 long" tells you nothing; "this room is 5 metres long" tells you the door is too narrow to fit a king-sized bed sideways.
A unit is a defined, agreed-upon reference quantity used to express the magnitude of a physical property. There are two flavours:
- Base units — defined by a standard. Since 2019, the kilogram is defined by fixing the value of Planck's constant. Before that it was an artefact in a vault in Paris. - Derived units — defined as a combination of base units. A newton is kg·m/s². A joule is kg·m²/s².
The SI (Système International) lists seven base units: kilogram, metre, second, ampere, kelvin, mole, candela. Every other unit you see — feet, calories, gigabytes — is either derived from one of these or a cultural/locale-specific variant.
When you convert between two units, you're really expressing the same quantity in two different reference frames. 1 metre and 3.281 feet describe the same length, just scaled by a factor — the conversion factor.
The rest of the articles in this section dig into specific traps: decimal vs binary prefixes for data, the affine mathematics of temperature, and why fuel-economy is the one quantity that flips upside down when you switch between metric and imperial.