Parts of a circle (GCSE resource)

See the full article on graphicmaths.com.

A circle is a curved shape. Every point on a circle is the same distance from the centre of the circle:

Radius of a circle

A radius of a circle is a line from the centre to any point on the edge of the circle. Since every point on the circle is an equal distance from the centre, you can draw a line from the centre in any direction and it will always form a radius.

The plural of radius is radii.

Diameter of a circle

The diameter of a circle is any line drawn between two points on the circle that passes through the centre of the circle:

The diameter is formed by 2 radii, so the length of the diameter is twice the length of the radius.

Circumference of a circle

The circumference of a circle is the length around the edge of the circle:

Formula for the circumference of a circle

The circumference of a circle is given by:

Where l is the circumference, r is the radius, and pi is a constant, equal to approximately 3.141592654. This equation can also be written as:

Where d is the diameter of the circle. This equation is true because the diameter is twice the radius.

Formula for the area of a circle

The area of a circle is given by: