Enter the radius to find a circle area, circumference, and diameter.
Everything from the radius
A circle is defined entirely by its radius, the distance from the center to the edge. From it you get the diameter by doubling, the circumference by multiplying by two pi, and the area by multiplying pi by the radius squared. The constant pi, roughly 3.14159, is the unchanging ratio of circumference to diameter.
A useful intuition: because area depends on the radius squared, doubling the radius does not double the area, it quadruples it. That square relationship explains why a slightly larger pizza is a much better deal than its diameter alone suggests.
The formula
Worked example
A circle with radius 5 has area pi x 25, about 78.54, and circumference about 31.42.
How to read your result
Every measure of a circle follows from its radius. Doubling the radius doubles the circumference but quadruples the area, since area depends on the radius squared.
Frequently asked questions
- What is pi?
- The ratio of a circle circumference to its diameter, about 3.14159.
- How do I find area from diameter?
- Halve the diameter to get the radius, then apply the area formula.
- What units does it use?
- Whatever unit you enter; area comes out in that unit squared.
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