How this calculator works
The circumference is the distance once around the circle. It is π times the diameter, or equivalently 2π times the radius, so the two modes are the same formula in different clothing. Negative measurements are rejected, and zero is allowed, giving a circumference of zero.
Formula
C = 2πr = πd, with π ≈ 3.141592653589793.
Worked example
A radius of 3 gives C = 2π × 3 ≈ 18.8496. A diameter of 10 gives C = π × 10 ≈ 31.4159.
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Frequently asked questions
Is circumference the same as area?
No. Circumference is a length around the edge; area measures the surface inside and is calculated with πr² on the circle area page.
How are the two modes different?
Only in what you type. The diameter is exactly twice the radius, so both modes produce the same circumference for the same circle.
Can I use this for an oval or ellipse?
No. The formula applies to a true circle. An ellipse has two different radii and needs a different perimeter approximation.