Perimeter

In mathematics, the perimeter is the distance around a two-dimensional shape or the border or outer boundary of a two-dimensional figure.

For example: the perimeter of this rectangle is 3+7+3+7 = 20

Perimeter Formulas

The basic formula for finding the perimeter is just to add the lengths of all the sides together

		Triangle Perimeter = a + b + c
		Square Perimeter = 4 × a a = length of side
		Rectangle Perimeter = 2 × (w + h) w = width h = height
		Quadrilateral Perimeter = a + b + c + d

If you want to put a border around the edge of a poster or a room, you need to find the perimeter.

To find the perimeter you add the measurements of all the sides.

Example: 4cm

3cm 3cm

4cm

The perimeter of this rectangle = 4 + 3 + 4 + 3 cm = 14cm

Example: 14m

10m 11m

16m

This children’s playground is to have a fence round the outside. How long will it be?

14 m +

11 m

16 m

10 m

51 m Answer 51 m long

Sometimes the diagram may not show all the side lengths. It’s easier if you write them on yourself.

Example:

15m

The length of this hall is 15m.

The width is 9m.

Write these measurements in: 15m

Perimeter = 15m

15m 9m 9m

9m 15m

48m The perimeter = 48m

Re-cap: the perimeter is the distance around the edge of a shape.

If the measurements you’re using have decimal fractions, check that you line up the decimal points when you add. Whole numbers are best written with a decimal point - e.g. 6cm can be written 6.0cm.

Example:

4.6cm

4.6

2.5cm 2.5

+ 2.5

14.2cm

The perimeter = 14.2cm

Complex Shapes

In more complex shapes, you still need to add all the side measurements.