Basic Units
Year 2 pupils often struggle when asked whether to measure a pencil in metres or centimetres, leading to answers like '0.15 metres' instead of '15 centimetres'. Teaching basic units requires systematic practice with familiar objects to build intuitive understanding of appropriate measurements.
Try it right now
Click “Generate a problem” to see a fresh example of this technique.
Why it matters
Mastering basic units forms the foundation for all measurement work throughout primary school and beyond. When children understand that a classroom door is 2 metres tall (not 200 centimetres), they develop number sense that prevents errors in real-world situations. A child buying 500 grams of apples at the greengrocer needs to recognise this is half a kilogram, whilst understanding that a medicine dose of 5 millilitres is a teaspoon, not 5 litres. These practical applications appear in Year 2 SATs questions and continue through GCSE mathematics. Without solid unit recognition, pupils struggle with conversion problems, leading to errors like calculating journey times in centimetres per hour instead of kilometres per hour.
How to solve basic units
Choosing Appropriate Units
- Length: mm (small), cm (hand-sized), m (room), km (distance).
- Mass: g (light), kg (everyday), tonnes (very heavy).
- Capacity: mL (spoon), L (bottle).
- Choose the unit that gives sensible numbers.
Example: A door is about 2 m tall (not 200 cm or 0.002 km).
Worked examples
You want to measure an eraser. Would you use cm, m, or km?
Answer: cm
- Think about the size of the thing → an eraser is about 5 cm — Picture an eraser in your hand or in your mind. Is it something you can hold? Something that fits on a table? That tells you it's small.
- Choose the right unit: small things use small units → Best unit: cm — Small lengths use cm (not metres -- that's for rooms). Light things use g (not kg -- that's for people). Small amounts of liquid use mL (not L -- that's for bottles). An eraser is about 5 cm, so cm is perfect.
- State the answer → cm — We measure an eraser in cm.
What unit would you use to measure the capacity of a water bottle?
Answer: L
- Think about the size of the thing → a water bottle is about 1.5 L — Picture a water bottle. Is it big enough to walk across? Heavy enough to carry with two hands? That tells you it's a medium-to-large thing.
- Big things use big units → Best unit: L — Big lengths use m or km (imagine measuring a classroom in mm -- you'd get a huge number!). Heavy things use kg. Large volumes use L. Using the right-sized unit keeps the number manageable.
- State the answer → L — We measure a water bottle in L. It's about 1.5 L.
How many mL are in 2 L?
Answer: 2000
- Remember: 1 L = 1000 mL → 1 L = 1000 mL — This is the key fact. Think of 1 L as a big box that contains 1000 smaller mL boxes inside it.
- Bigger to smaller = multiply → 2 x 1000 = ? — We have 2 big units. Each one 'unpacks' into 1000 small units. More small pieces means multiply. Like opening 2 bags of 1000 sweets -- you get lots of sweets!
- Calculate → 2 x 1000 = 2000 — 2 x 1000 = 2000. So 2 L = 2000 mL.
Common mistakes
- Using inappropriate units for size, such as measuring a rubber in metres instead of centimetres, giving answers like '0.05 metres' rather than '5 centimetres'
- Confusing mass and capacity units, writing that a bottle of milk weighs '1 litre' instead of stating its capacity is '1 litre' and mass is about '1 kilogram'
- Mixing up grams and kilograms for everyday objects, claiming a bag of sugar weighs '1000 grams' instead of recognising this equals '1 kilogram'
- Converting incorrectly between units, calculating 3 metres = 30 centimetres instead of 300 centimetres by forgetting the factor of 100