Radius input
Radius is half the diameter. If you know diameter, divide it by 2 before entering it here.
Example
With a radius of 5, the diameter is 10, the circumference is about 31.42 and the area is about 78.54 square units.
How to use the result
Use circumference when you need the distance around the outside edge. Use area when you need the space inside the circle, such as a round floor, tabletop or garden bed.
Common mistakes
Area and circumference use different units. Circumference is a length; area is square units. Do not compare the two numbers as if they measured the same thing.
Last reviewed: 2026-05-17