Circles

A circle is a closed plane shape which has no corners or line segments. Every point on the boundary of a circle is the same distance from the center. Look at the green circle below.

Important things to know about circles

The distance all around a circle is known as the circumference of the circle. The circumference can also be described as the perimeter of a circle or the boundary of a circle.

Any straight line which goes from the center of a circle to the circumference is called the radius of the circle. In the diagram above, the radius is the red line.

The distance across the circle, through the center is called the diameter of the circle. In the diagram above, the diameter is shown as a blue line.

For any circle, the diameter is twice the length of the radius. The radius is half the length of the diameter. Therefore, if the radius of a circle is 4cm, the diameter of that circle is 8 cm. If the diameter of a circle is 14 meters, the radius of that circle is 7 meters.

Note: Diameter = radius x 2  and Radius = diameter ÷ 2

The ratio of the circumference of a circle to the diameter of that circle is called pi. Pi is always about 3.14 no matter how big or small a circle is. Pi can be used to calculate the circumference and area of a circle.