The perimeter is the total distance around the outside of a shape. For a shape with right-angled corners, first make sure you know every side. A missing side can be worked out from the others: opposite directions must balance, so the short sides across the top add up to the long side across the bottom. Then start at one corner and add every side as you go round, ticking each one off. Don't multiply: that gives the area.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- perimeter
- the distance all the way around a shape
- composite shape
- a shape made by joining simpler shapes
Worked examples
An L-shape has sides of 6 m, 3 m, 4 m, 5 m, 10 m and 8 m. What is its perimeter?
- 1.Add every side.
- 2.6 + 3 + 4 + 5 + 10 + 8 = 36 m.
= 36
An L-shape has outer sides of 10 m and 8 m and a corner cut out. What is its perimeter?
- 1.The short sides across the top add up to 10, and the sides down add up to 8.
- 2.10 + 10 + 8 + 8 = 36 m.
= 36
A U-shape is 12 m by 6 m with a gap 3 m deep. What is its perimeter?
- 1.The outside rectangle gives 12 + 6 + 12 + 6 = 36.
- 2.The gap adds two more sides: 36 + 3 + 3 = 42 m.
= 42
For parents
Method: Find every side, then add
What to say: How many sides does the shape have? Do you know every one of them?
Common mistakes:
- Adds only the sides that have numbers on them, leaving out the unlabelled sides.
- Finds the perimeter of the rectangle around the shape, missing the extra sides inside the gap.
- Misses one of the short sides at the cut-out corner when adding.
- Works out the area (space inside) instead of the perimeter.