# Derived units

These are derived from the base units as some meaningful combination. Some have special names whereas others are known by the elements that make them up and their mathematical relationship. Only a select few are listed here.

The unit names are only shown where applicable.

## SI derived units

Symbol Unit name What it measures
m2 square metre
Area
m3 cubic metre
Volume
m/s metre per second Speed or velocity
m/s2 metre per second squared
Acceleration
Rate of change of speed (can be increasing or decreasing) e.g. if a moving object increases its speed by 1 m/s for every second of time that passes, then it is accelerating at the rate of 1 m/s2
kg/m3 kilogram per cubic metre Density
N newton
Force
The force which will accelerate a mass of 1 kg at a rate of 1 m/s2
Pa pascal
Pressure = 1 N/m2
J joule
Energy
The energy required to accelerate a mass of 1 kg at a rate of 1 m/s2 over a distance of a metre (= 1 N m) Also tied to electrical units (see below). The kJ is the same as that used in nutrition data.
W watt
Power
Rate of expenditure of energy equal to 1 joule/second (J/s)
The familiar unit used with electrical appliances, but note the mechanical origin.
°C degree Celsius
Temperature
0 °C = 273.15 K
Based on the temperatures at which water freezes (0 °C) and boils (100 °C at normal atmospheric pressure)
As used in weather forecasts, heating thermostats and ovens, etc.
Hz hertz
Frequency (same as cycle/second)
The unit used for such things as radio channels, sound (pitch), and computer processor speed.