Concept

 A circle is a round, two-dimensional shape. All points on the edge of the circle are at the same distance from the center.

The radius is the distance from the center outwards.

The diameter goes straight across the circle, through the center.

The circumference is the distance once around the circle.

Rule

Radius is half of the diameter. To find the radius divide the diameter by 2.
Radius = Diameter2 or r = d2

The diameter is twice the radius. To find the diameter, multiply the radius by 2.
Diameter = 2 × Radius or d = 2r

To find the circumference, multiply the value of pi (π) with the diameter or multiply the value of pi (π) with the double of the radius. 

Circumference = π × Diameter or C = πd
Circumference = 2 × π × Radius or C = 2πr 

Example

Choose the correct label for the indicated part of the circle.

Solution

The circumference is the distance once around the circle.

Pre-requisite Skills

None