Definition
Compound unit
A compound unit combines units by multiplication or division to describe one quantity.
What it means
The unit tells how two measurements are connected. In kilometres per hour, the word per means divide: distance is compared with time. When converting, the number and every unit part must stay consistent.
Remember it
Read the unit as a relationship: kilometres per hour means kilometres for each hour.
Example
A cyclist travels 36 kilometres in 2 hours. The speed is 36 ÷ 2 = 18 kilometres per hour, written 18 km/h.
Watch for
A single unit such as cm is not a compound unit. The metrology term derived unit is broader and is not always interchangeable with compound unit.
Common mix-up
Do not convert only the number; check every unit in the combined unit.
Exact meaning
A compound unit is formed by combining units through multiplication or division, often to measure a rate or a derived quantity. Examples include metres per second (m/s) and grams per cubic centimetre (g/cm³).