By the end of year 3 children are expected to know that: 1 kilometre = 1 000 metres 1 metre = 100 centimetres 1 kilogram = 1 000 grams 1 litre = 1 000 millilitres They are also expected to recognise half units, such as half a kilometre and that this can be written as ½ km, or 0.5 km or 500 m. Problems involving comparisons continue in length, capacity and mass, and once again most of these should be on a practical basis. One of the most useful benefits of the metric system is the relationship between the units: a litre of water can be contained in a 10 cm cube (1 000 cubic cm) and has a mass of 1 kg. Hence a 1 cm cube of water has a mass of 1 gram and is known as 1 ml. Choosing suitable units continues from year 2, with many similar activities, but a greater emphasis on standard units and mathematical language. It is an excellent idea to have a collection of pots, cartons etc which show the capacity and wrappers which show weight. These can be grouped in various ways and comparisons made. Choosing the correct unit to measure is important and practice can be given orally on these questions - hold up a pot and ask what you would measure the capacity in as part of the daily mental arithmetic. In Year 3 children measure and draw lines to the nearest half centimetre. It is important that they have rulers which include millimetres and that they understand that 0.5 is the same as a half. Much more can be found in our measurement for Year 3 pages here.