Circle Calculator
0 area
| Property | Formula | Value |
|---|---|---|
| Radius (r) | — | |
| Diameter (d) | 2r | |
| Circumference (C) | 2πr | |
| Area (A) | πr² |
How the Circle Calculator Works
Every measurement of a circle — radius, diameter, circumference, and area — can be derived from just one of them because they're all tied together by the constant π (pi, approximately 3.14159265). Enter any single known value and this calculator solves for the rest using the standard formulas: diameter d = 2r, circumference C = 2πr (equivalently C = πd), and area A = πr². If you start from circumference or area instead of radius, the calculator simply solves each formula backward (r = C / (2π), or r = √(A / π)) before recomputing everything else.
A Common Mix-Up: Radius vs. Diameter
The most frequent error is plugging a diameter into a formula that expects a radius (or vice versa), which throws every downstream result off by a factor of 2 (or 4, for area, since area scales with the square of the radius). If your circumference or area comes out looking exactly double or quadruple what you expected, double-check whether the value you entered was really the radius and not the diameter.
Related Geometry Tools
For circles that are actually part of a triangle or a 3D shape, see the area calculator for other 2D shapes, or the volume calculator if you're working with a sphere or cylinder built from this circle.
Frequently Asked Questions
How do I find the area of a circle if I only know the diameter?
First divide the diameter by 2 to get the radius, then apply A = pi r squared. For example, a diameter of 10 gives a radius of 5, so the area is pi times 5 squared, about 78.54 square units. This calculator does that conversion automatically when you select "Diameter" as your known value.
Why do my circumference and area seem off by a factor of 2 or 4?
This almost always means a diameter was entered where a radius was expected (or vice versa). Since circumference scales linearly with radius (C = 2 pi r) and area scales with radius squared (A = pi r squared), mixing up radius and diameter throws circumference off by 2x and area off by 4x. Double-check which value you selected in the "What do you know?" field.