Parts of a circle
A circle is a round shape where every point on the edge is exactly the same distance from the middle. We see circles in wheels, coins, plates and clocks. The middle point is the centre. A straight line from the centre to the edge is the radius. A straight line right across the circle and through the centre is the diameter.
Key idea
Diameter = 2 × radius and radius = diameter ÷ 2. The distance all the way around the circle is the circumference.
Finding radius and diameter
Worked example
A circle has a radius of 5 cm. What is its diameter? Diameter = 2 × radius = 2 × 5 = 10 cm. If instead the diameter were 14 cm, the radius would be 14 ÷ 2 = 7 cm.
Remember
- The diameter is twice the radius.
- The radius is half the diameter.
- Circumference = the distance around; an arc is part of it.
- To go from radius to diameter multiply by 2; to go from diameter to radius divide by 2.