# Perimeter

A Perimeter is a one dimensional measurement. It is a length measured usually in metric units such as CM M KM.

When calculating the perimeter of a shape, remember that you are looking for a measurement all the way around its edge. So simply add up all the sides of any shape. You must also remember all units of measurement are the same e.g. CM.

**Perimeter of a Circle**

The perimeter of a circle is known as the circumference. To calculate the circumference of a circle you can use one of two formulae, either: C = 2πr or C = πd. In these formulae r is the radius and d is the diameter.

Looking at example 1 below you can calculate the circumference as follows:

C = 2 X π X r (6.28318 x 4.2) = 26.39 (to 4 significant figures)

Looking at example to below you can calculate the circumference as follows:

C = π X d (3.14159 x 10) = 31.42 (to 4 s. f.)

To calculate the perimeter of an arc (part of a circle) from the angle you can calculate the fraction of the circle.

In the example above we can work this out as follows:

60/360 x 2π x 4 = 4/3 π cm

= 4.19cm (to 3 s.f.) The perimeter of the whole sector is therefore 4.19 + 8 = 12.19 (4 s.f.).

**The perimeter of a rectangle**

To calculate the perimeter of a rectangle, just add all the sides together.

In the example above the answer is 26cm.