Perimeter

In a Nutshell

The perimeter of a shape is the total distance around its outside. Add up the lengths of all the sides.

Perimeter means the total length of the boundary of a flat shape. To find it, simply add together the lengths of every side.

For a rectangle with length $l$ and width $w$ :

$\text{Perimeter} = 2l + 2w = 2(l + w)$

For a square with side length $s$ :

$\text{Perimeter} = 4s$

For any other shape, add up all the individual side lengths. Make sure every side is in the same unit before adding.

Press "Next side" to trace around the shape. Each side lights up and its length is added to the running total.

Watch it work

Question: Find the perimeter of a rectangle that is 12 cm long and 5 cm wide.

Have a go

Q1. Find the perimeter of a square with side length 9 cm.

Q2. A triangle has sides 7 cm, 10 cm and 13 cm. What is its perimeter?

Q3. A rectangle has a perimeter of 40 cm and a length of 14 cm. Find the width.

Q4. An L-shaped garden is made from two rectangles. The outer dimensions are 10 m by 8 m, with a 4 m by 3 m piece cut from one corner. Find the perimeter of the L-shape.

Q5. An equilateral triangle has the same perimeter as a square of side 6 cm. What is the side length of the triangle?

Bonus: circles and pi

A circle's perimeter is called its circumference. But how long is it? It turns out the circumference is always exactly π times the diameter. Watch the circle below roll along a line to see why.

Radius: 35

Press Roll to see the circle complete one full turn. The coloured trail on the line is the circumference — and it always lands between 3d and 4d, at exactly πd. Change the radius with the slider: the circle gets bigger or smaller, but the ratio stays the same.