The perimeter of the polygons

What is the perimeter?

The perimeter of a shape is the total length of its outline. Imagine walking all the way round a football pitch: the distance you cover is the perimeter of the pitch! It is always expressed in a unit of length (cm, m, km…), never in square units.

Perimeter of a rectangle and a square

For a rectangle with length L and width l: P = 2 × (L + l)

For a square with side length c, all four sides are equal: P = 4 × c

Perimeter of a triangle

Simply add the three sides together: P = a + b + c

Practical example

Shape	Dimensions	Calculation	Perimeter
Rectangle	L=7 cm, l=4 cm	2 × (7+4)	22 cm
Square	c=5 cm	4 × 5	20 cm
Triangle	3 cm, 4 cm, 5 cm	3+4+5	12 cm

Pitfall to avoid

Don’t confuse perimeter with area! Perimeter is measured in cm, m… (a single dimension), whilst area is measured in cm², m²… Another common mistake is forgetting to multiply by 2 for the rectangle, by counting only two sides instead of four.