# Compound measures

**Compound measures**

Density is an example of a **compound measure**, it is a measure of the mass of an object per unit volume. Density is usually measured in g/cm³ or kg/m³ etc.

For example: A density of 1.5 g/cm³ means that every 1 cm³ of the material has a mass of 1.5 grams.

The density of a given material is calculated using the following formula:

A change in **constant rate** is when one measurement changes in direct proportion with another. For example, speed is measured in km/h or mph and if calculated as follows:

**Average speed**

As in some circumstances, speed is not always constant, the average speed may need to be calculated. The average speed is calculated as follows:

**Converting compound units**

When converting compound units, it is usually carried out in several steps. For example:

50 mph stands for 50 miles per hour.

If 50 miles is being converted into metres and seconds, the following steps are taken:

50 miles = 50 x 1.6 km (1 mile = 1.6 km)

50 miles = 80 km

50 miles = 80,000 metres

1 hour = 60 minutes

1 hour = 3,600 seconds

To convert 50 mph to m/s the following steps are carried out:

50 mph = 80,000 / 3,600

50 mph = 22.22 m/s (to 2 d.p.)