Choosing Metric Units for Volume and Capacity
26 October 2026
Choosing metric units for volume and capacity
Understanding which metric units to use for measuring volume and capacity is a key skill in 11+ Maths. Children need to know the difference between volume and capacity, and when to use units like millilitres, litres, and cubic centimetres.
Volume or capacity
Volume measures the amount of space a 3D object takes up, usually in cubic centimetres (cm³) or cubic metres (m³). Capacity is about how much liquid a container can hold, measured in millilitres (ml) or litres (l). For example, the volume of a box is given in cm³, but the capacity of a water bottle is in ml or l.
When to use each unit
Use millilitres (ml) for small amounts of liquid, like a teaspoon or a small medicine bottle. Use litres (l) for larger amounts, such as a drinks bottle or a bucket. Use cubic centimetres (cm³) for the volume of solid objects, especially when measuring or calculating using length, width and height in centimetres.
Key insight: Look at the size and type of object
If the question is about a container for liquid, think capacity (ml or l). If it is about the space inside a box or a shape, think volume (cm³ or m³). The context of the question usually gives you a clue about which unit to choose.
Worked example
Choosing the right unit for a water bottle
Step 1: Read the question. A water bottle holds 750 of something. What is the most suitable unit: millilitres, litres, or cubic centimetres?
Step 2: Think about the object. A water bottle is a container for liquid, so we are measuring capacity.
Step 3: Consider the size. 750 is a number that fits well with millilitres (ml), as most bottles are labelled in ml or l.
Step 4: Check the alternatives. 750 litres would be far too large for a bottle, and 750 cm³ is less common for liquids.
The correct answer is 750 millilitres (ml).
Tips for recognising the right unit
If the answer would be a very large or very small number in one unit, check if another unit makes more sense. For example, use litres for a bath (not millilitres), and millilitres for a spoonful of medicine (not litres).
Supporting your child
Practise spotting the difference between volume and capacity in real-life objects at home. Look at packaging and labels to see which units are used, and talk through why that unit is the best choice for each item.
