Instant circle properties from radius, diameter, circumference, or area
Enter Any One Value
Type into any field — the other three are calculated instantly. All values share the same unit (e.g. cm, m, in).
All Circle Properties
= 3.14159265358979…
Formula Reference
The Circle Calculator lets you solve any circle property in one step. Enter a radius, diameter, circumference, or area and the tool instantly derives the remaining three using the exact formulas C = 2πr and A = πr². Whether you are a student checking homework, an engineer sizing a pipe, or a designer scaling a logo, this free tool gives precise results with no rounding surprises.
Use the formula C = 2 × π × r. For example, a circle with radius 5 has a circumference of 2 × π × 5 ≈ 31.4159 units. Just type the radius into this calculator and the circumference is shown instantly.
Rearrange the area formula A = πr² to get r = √(A / π). Enter the area in the Area field and the calculator solves for radius, diameter, and circumference automatically.
The radius (r) is the distance from the center of a circle to its edge. The diameter (d) is the straight-line distance across the circle through its center, so d = 2r. They are directly proportional: doubling the radius doubles the diameter.
Yes. The formulas are unit-agnostic. Enter values in centimeters, meters, inches, or any consistent unit and the results use the same unit (area results are in that unit squared). Just be consistent — don't mix cm for one field and inches for another.