The perimeter is the total distance all the way around the outside of a shape. A rectangle has two lengths and two widths, so add all four sides. For a 5 cm by 3 cm rectangle: 5 + 3 + 5 + 3 = 16 cm, which is the same as 2 × (5 + 3). Only two measurements are printed, but each one is used twice. Multiplying the two sides together gives the area instead, not the perimeter.
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 around the outside of a shape
- length
- the longer side of a rectangle
- width
- the shorter side of a rectangle
Worked examples
Perimeter of 5 cm by 3 cm
- 1.Add four sides: 5 + 3 + 5 + 3.
- 2.= 16 cm.
= 16
Perimeter of 7 cm by 4 cm
- 1.2 × (7 + 4) = 2 × 11.
- 2.= 22 cm.
= 22
Perimeter of 10 cm by 6 cm
- 1.2 × (10 + 6) = 2 × 16.
- 2.= 32 cm.
= 32
For parents
Method: Trace a finger around the border while counting.
What to say: Have them label all four sides before adding — each printed side appears twice.
Common mistakes:
- Multiplying the sides (that is the area).
- Adding only the two given sides and forgetting the other two.