Units & Conversions
Converting between metric units (mm, cm, m, km; g, kg; ml, l) and using approximate imperial equivalents.
What is Units & Conversions?
Being able to convert between different units of measurement is a key skill in the 11+ exam. You need to know the relationships between metric units and be able to use approximate imperial equivalents.
Questions may ask you to convert between units, compare measurements in different units, or solve problems that require conversion as part of the working.
Step-by-Step Method
Know the key metric conversions
10mm = 1cm, 100cm = 1m, 1000m = 1km. 1000g = 1kg. 1000ml = 1l.
To convert to a smaller unit, multiply
Going from km to m means the number gets bigger: 2.5km = 2,500m (multiply by 1,000).
To convert to a larger unit, divide
Going from g to kg means the number gets smaller: 3,500g = 3.5kg (divide by 1,000).
Know approximate imperial equivalents
1 inch is about 2.5cm. 1 foot is about 30cm. 1 kg is about 2.2 pounds. 1 litre is about 1.75 pints. 1 mile is about 1.6km.
Always check your units in the answer
Make sure your answer is in the units the question asks for.
Worked Examples
Convert 3.5km to metres.
Working
- 1km = 1,000m.
- 3.5 x 1,000 = 3,500m.
Convert 4,500g to kg.
Working
- 1kg = 1,000g.
- 4,500 / 1,000 = 4.5kg.
Convert 2.5 litres to ml.
Working
- 1 litre = 1,000ml.
- 2.5 x 1,000 = 2,500ml.
Common Mistakes
Multiplying when you should divide, or vice versa.
Think: am I going to a SMALLER unit (multiply) or a BIGGER unit (divide)?
Confusing the conversion factors (e.g. thinking 1m = 1,000cm).
1m = 100cm (not 1,000). Write the conversions out and double-check.
Forgetting to convert units before comparing measurements.
You cannot compare 3.5km with 3,200m directly. Convert one so both are in the same unit.
Top Tips
- The pattern for metric units: milli (1/1000), centi (1/100), kilo (1000x). This works for metres, litres and grams.
- When multiplying or dividing by 10, 100 or 1,000, just move the decimal point.
- Draw a quick conversion ladder: km – m – cm – mm. Each step down means multiply by 10 or more.
- For imperial questions, you only need approximate values – do not try to be exact.
Ready to practise?
Put these techniques into action with our free practice papers.
Practise Maths Questions