Compound Measures

Number

A compound measure is built from two different measurements rather than one
It expresses a rate: how much one quantity changes for each unit of the other
The word "per" signals the division, and the units say which way round it goes
- Speed measures distance per unit of time
- Population density measures people per unit of area
- Fuel consumption measures fuel used per unit of distance

The units of a compound measure state its formula, which removes any need to memorise them separately
A unit written as a/b means the quantity measured by a divided by the quantity measured by b
- g/cm³ is grams divided by cubic centimetres, so it is mass divided by volume
- people per km² is a count divided by an area