The standard unit for measuring length is the metre, m.
The metre is part of the metric system or ' The International System of Units (SI)' which was established in 1960 and is derived from the distance that light travels in a vacuum over a very small time interval - 1/299,792,458 of a second!
1 metre is about the height of a door handle in your house.
Other metric units of length are derived from the metre. Common units are:
kilometre, km = 1000 metres
millimetre, mm = 1/1000 of a metre - about the thickness of a 5p coin.
centimetre, cm = 10mm = 1/100 of a metre - about the width of a little fingernail.
In the UK we use a combination of Metric and Imperial units, which means that your child has to understand and be able to compare both types of measurement.
Imperial units of length are miles, yards, feet and inches.
One inch is approximately 2.5 cm - about the diameter of a 2p coin.
One foot = 12 inches - approx 30cm - about the height of a wine bottle.
One yard = 3 feet = 36 inches - approx 92 cm, a bit less than a metre.