In the same way that areas are measured by how many squares fit within a shape, volumes are found by the number of cubes that fit inside.
Volumes are normally measured in cubic centimetres or cubic metres, but you could use anything. As with area, it is easy to get confused between volumes.
One cubic centimetre (1 cm3) is a cube 1 cm along each side:
This happens to be the same volume as a milli-litre (ml), 1000 of which make up a litre.