What are imperial units?

What are imperial units?
How children learn about imperial units of measurement in primary school, plus metric to imperial conversion tables.
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What are imperial units?

When we measure the length, mass or capacity of something, we need a standard unit to measure it in. 

In the past, imperial units of measurement were used in the UK. The imperial system has gradually been replaced by the metric system, which is easier to understand as it deals with tens, hundreds and thousands.

Lengthmillimetre, centimetre, metre, kilometreinch, foot, yard, mile
Massmilligram, gram, kilogramounce, pound, stone
Capacitymillilitre, centilitre, litrepint, gallon

Measuring with imperial units

This table tells you all you need to know about imperial units and how they relate to metric units:

Imperial unitHow many smaller imperial units are in this one?What is this approximately in metric units?
1 inchNone2.5cm
1 foot12 inchesRoughly 30cm
1 yard3 feet91.4cm (nearly one metre)
1 mile1760 yards1.6 kilometres
1 ounceNone28 grams
1 pound16 ounces453 grams (nearly half a kg)
1 stone14 pounds6.4 kilograms
1 pintNone568 ml (just over half a litre)
1 gallon8 pints4.5 litres

When do children learn about imperial units in primary school?

In Key Stage 1, children will be introduced to standard and non-standard measures and required to use vocabulary such as taller, shorter, longer, heavier, lighter, empty, half-full and full.

In Year 3, they will be introduced to the metric units of measurement and will be asked to measure length, mass and capacity and then record their measurements. 

By Year 4, they will be expected to convert metric measurements; for example, they may be told that a door is 2 metres tall and then asked to give this measurement in cm (200 cm). They continue this work in Year 5, going onto harder tasks, such as converting 0.3cm to mm (3mm) or 3.7kg to grams (3700g).

In Year 5, imperial units will be introduced. Children will be asked to find approximate equivalences between the metric and imperial units. The curriculum states that they need to know about inches, pounds and pints.

In Year 6, children will be asked to convert between miles and kilometres. It is possible your child's teacher will broach this by telling them that 5 miles equals approximately 8 kilometres; therefore, if you are given a journey in kilometres and want to convert it to miles, you divide by 8 and then multiply by 5. If you are given a journey in miles and want to convert it to kilometres, you divide it by 5 and then multiply by 8.

Please do not worry if this all seems totally baffling! Imperial units represent a very small part of the maths curriculum, and there is likely to be only one question in the SATs on it.  It is however, very important that your child is confident with the metric system (reading scales, converting measurements, calculating with measurements), so spend plenty of time on this before attempting to think about imperial units.