What is perimeter?
Year 3 measures the perimeter of simple shapes, Year 4 of rectilinear shapes, and Year 6 sees that shapes with the same area can have different perimeters. Below: what it means, a worked example, the mistake to watch for and an 11+ question.
The answer
Perimeter is the total distance all the way round the outside of a shape.
What it means
Add the lengths of all the sides. A rectangle 8 cm long and 5 cm wide has a perimeter of 8 + 5 + 8 + 5 = 26 cm.
Perimeter is a length, so it is measured in cm, m or km, never in square units.
A worked example
Find the perimeter of a square with sides of 7 cm
- A square has 4 equal sides.
- 4 × 7 = 28, so the perimeter is 28 cm.
The common mistake
Missing sides on an L-shape. Count the sides first (an L-shape has 6) and work out any lengths that are not labelled before adding.
How it comes up in the 11+
A square has a perimeter of 36 cm. What is its area?
Answer: 81 cm²
- The 4 sides are equal, so each is 36 ÷ 4 = 9 cm.
- Area = 9 × 9 = 81 cm².
Try it yourself
It opens on the 8 cm by 5 cm rectangle from above. Count the edges round the outside, then tap squares to change the shape.
Draw a shape on squared paper to see its area, the squares inside, and its perimeter, the edges round the outside.
Related words and practice
More maths words
Questions parents ask
What units is perimeter measured in?
Units of length, like mm, cm, m and km. Area uses square units, like cm².
Can two shapes have the same area but different perimeters?
Yes. A 1 cm by 12 cm rectangle and a 3 cm by 4 cm rectangle both have an area of 12 cm², but their perimeters are 26 cm and 14 cm.
Knowing the words is half of an 11+ maths question
11+ papers use maths vocabulary without explaining it. The free diagnostic checks maths, English and both kinds of reasoning, and shows what to practise next. No card is required.