Choosing appropriate units

The commonly used metric units of length include:

  • kilometres (\({km}\))
  • metres (\({m}\))
  • centimetres (\({cm}\))
  • millimetres (\({mm}\))

The commonly used metric units of mass include:

  • gram (\({g}\))
  • kilogram (\({kg}\))
  • tonne (\({t}\))
  • milligram (\({mg}\))

The commonly used metric units of capacity include:

  • litre (\({l}\))
  • millilitre (\({ml}\))
  • cubic centimetre (\({cc}\))


To find a suitable unit to measure something in, you should first estimate how big it is. So, for example, do you think it's better to measure the distance between London and Manchester in metres or kilometres, or should the length of an insect be measured in metres or millimetres?

Imagine the length of the objects you are estimating or imagine measuring them in relation to other objects. For example:

  • Imagine a metre ruler next to a tall man. You would expect him to be taller than \({1}\) ruler, but not as tall as \({3}\) rulers.
  • Imagine a bag of sugar on some weighing scales. You would expect the needle to move round a little, but not to whizz round to \({50~kg}\).
  • Imagine a \({1}\)-litre bottle of cola. You would expect it to hold less than a bucket full of water.

Q1. Which unit of measurement (\({km}\), \({m}\), \({cm}\)or\({mm}\)) would you use for the following?

a) the length of the span of your hand

b) the length of a football pitch

c) the thickness of a notebook

d) the distance from London to Manchester


a) Is the height of a tall man likely to be \({90}~{cm}\), \({180}~{cm}\) or \({360}~{cm}\)?

b) Is the weight of a bag of sugar about \({1}~{kg}\), \({10}~{kg}\) or \({50}~{kg}\)?

c) Does a kettle hold roughly \({200}~{ml}\), \({1}~{l}\) or \({10}~{l}\) of liquid?


a) \({cm}\)

b) \({m}\)

c) \({mm}\)

d) \({km}\)


a) \({180}~{cm}\)

b) \({1}~{kg}\)

c) \({1}~{l}\)