What are radius and diameter?

The radius of a circle is the distance from its centre to any point on the edge. The diameter is the distance straight across the circle through the centre. The diameter is always twice the radius, so if you know one you can find the other.

Examples

• A circle with a radius of 3 cm has a diameter of 6 cm.
• A diameter of 10 cm means a radius of 5 cm.
• Diameter = 2 × radius.

Quick facts

• The diameter is always double the radius.
• Every radius of the same circle is equal in length.