Perimeter is the distance all the way around the edge of a flat shape. If you walked around the very edge of a football pitch, the total distance you walked would be its perimeter.
Why this matters: You use perimeter in real life more than you think. How much fencing for a garden? How much ribbon to go round a picture frame? How much braid to sew round the edge of a prayer mat? All of these are perimeter.
How to find a perimeter
Just add up the length of every side. Go round the shape once and do not miss a side. Keep the units the same (all cm, or all m).
Worked example: A rectangle picture frame is 20 cm long and 15 cm tall. It has 4 sides: 20 + 15 + 20 + 15 = 70 cm. So you need 70 cm of ribbon to go all the way round.
A quick way for rectangles
A rectangle has two long sides the same and two short sides the same. So you can add the long and short, then double it.
| Length | Width | Working | Perimeter |
|---|---|---|---|
| 20 cm | 15 cm | (20 + 15) × 2 | 70 cm |
| 6 m | 4 m | (6 + 4) × 2 | 20 m |
Put these steps in order to find a rectangle's perimeter:
- Find the length and the width
- Add the length and width together
- Multiply the answer by 2
Compound shapes
Some shapes are made of straight sides but are not a simple rectangle, like an L-shape. Do not worry! The rule is the same: add up every single side going round once.