How this calculator works
A circumference is the distance once around a circle. Dividing it by 2 pi gives the centre-to-edge radius. If the circumference is already written as k pi, the radius is simply k/2 in the same length unit. The remaining circle measurements follow from that radius. A zero measurement is accepted as a degenerate circle and produces zeros throughout. All geometric lengths are nonnegative.
Formula
C = 2 pi r; r = C/(2 pi); d = 2r; A = pi r^2. If C = k pi, r = k/2.
Worked example
A circumference of 6 pi metres is about 18.8496 metres. Dividing by 2 pi gives radius 3 metres, diameter 6 metres and area 9 pi square metres. A circumference of 10 pi metres gives radius 5 metres.
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Frequently asked questions
What do I enter for a circumference of 12 pi centimetres?
Choose the pi-coefficient mode, enter 12 and select centimetres. The resulting radius is 6 centimetres.
Does changing units change the circle?
No. Lengths convert to a common base before calculation. Output selectors let you view the same radius, circumference or area in the supported units.
Can I use this formula for an ellipse?
No. An ellipse has two semi-axes and its perimeter is not generally 2 pi times either one.