Level 2 · Ages 8–11
Read it again any time, including while you work through the exercises.
Imagine an ant walking all the way around a shape along its edge, never cutting across. The total distance it travels is the perimeter of the shape!
For a polygon (a shape with straight sides), it is simple: the perimeter is the sum of the lengths of all the sides. A triangle with sides of 3 cm, 4 cm and 5 cm has a perimeter of 3 + 4 + 5 = 12 cm.
Some shapes have clever shortcuts. The square has 4 identical sides: instead of adding the same number 4 times, we multiply the side by 4. Perimeter of a square = 4 × side.
The rectangle has 2 equal lengths and 2 equal widths. So its perimeter is 2 times the sum length + width. Perimeter of a rectangle = 2 × (length + width).
One last pro tip: always check that all lengths are in the same unit before adding, and do not forget the unit in your answer!
Compute the perimeter of a rectangle with length 5 cm and width 3 cm.
P = 2 × (L + l)
A rectangle has 2 lengths and 2 widths: I take 2 times the sum length + width.
5 + 3 = 8
First I add the length and the width: 5 + 3 = 8.
2 × 8 = 16
Then I multiply by 2, because each side appears twice.
Result: The perimeter is 16 cm.
Compute the perimeter of a square with side 7 cm.
P = 4 × côté
The square has 4 identical sides: I multiply the side by 4.
4 × 7 = 28
I compute: 4 times 7 equals 28.
Result: The perimeter is 28 cm.