Circle Calculator
Enter one measurement of a circle and get the rest.
The four measurements
A circle is fully described by any one of its radius, diameter, circumference or area. The others follow from these formulas: d = 2r, C = 2πr and A = πr².
Example
A circle with a radius of 5 has a diameter of 10, a circumference of about 31.416 and an area of about 78.540.
Pi (π) is about 3.14159, the ratio of any circle’s circumference to its diameter. Units follow what you enter: lengths stay in your unit and the area is in square units.
Frequently asked questions
How do I find the radius from the area?
Divide the area by π and take the square root.
Do I need to enter units?
No. Use the same unit throughout and read the results in that unit, with area in square units.
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