Radius and diameter
Radius is the distance from the center to the edge. Diameter is twice the radius. Area measures the surface inside the circle, so the result is shown in square units.
Example
If the radius is 5, the area is pi x 5 x 5, or about 78.54 square units. The diameter is 10.
Common mistakes
Do not enter diameter as radius. If you measured across the full circle, divide that value by 2 first. Also keep units consistent before comparing circle area with another shape.
Limitations
This calculator assumes a perfect circle. Real-world measurements such as gardens, pipes or round tables may need allowance for measurement error or irregular edges.
Last reviewed: 2026-05-17