Perimeter · Grade 3
In Grade 3 you learn: perimeter of shapes; finding a missing side from the perimeter; shapes with the same area and different perimeters.
The perimeter of a shape is the distance all the way round it: add the lengths of all its sides. For a rectangle, perimeter = 2 × (length + width). Perimeter is a length, so it is measured in cm, m or km.
Key facts
- Perimeter is the distance all the way round a shape: add up all its sides.
- A square has 4 equal sides, so its perimeter is 4 × the side.
- Shapes with the same area can have different perimeters.
Worked examples
Example 1
Find the perimeter of a rectangle 6 cm long and 4 cm wide.
Answer: 20 cm
Example 2
A triangle has sides of 5 cm, 7 cm and 8 cm. What is its perimeter?
Answer: 20 cm
Example 3
A square has sides of 9 cm. What is its perimeter?
Answer: 36 cm
Example 4
A rectangle has a perimeter of 30 cm. It is 9 cm long. How wide is it?
Answer: 6 cm
Example 5
Two rectangles both have an area of 24 cm². One is 6 cm by 4 cm; the other is 12 cm by 2 cm. Which has the longer perimeter?
Answer: 12 cm by 2 cm (28 cm, not 20 cm)