Circle Calculator
π = 3.14159265358979323846...
How to use
- Choose which measurement you already know: radius, diameter, circumference or area.
- Type the value. The other three measurements appear straight away.
- Use the decimals menu to set the precision. For a radius or diameter, the circumference and area are also shown as exact multiples of π.
How it works
Diameter d = 2r, where r is the radius.
Circumference C = 2πr = πd. Area A = πr².
Working backward, r = C ÷ 2π, or r = √(A ÷ π). The calculator first finds the radius from whatever you enter, then works out the rest from it.
π is the ratio of any circle's circumference to its diameter, about 3.14159265358979. The calculation uses the full precision of your browser, and only the displayed result is rounded. Units follow what you enter: a radius in centimeters gives an area in square centimeters.
FAQ
How do I find the area of a circle from its diameter?
Halve the diameter to get the radius, then use A = πr². For a diameter of 10, the radius is 5 and the area is 25π, about 78.54.
How do I find the radius from the circumference?
Divide the circumference by 2π. A circle with a circumference of 31.4159 has a radius of 5.
What does the exact answer in terms of π mean?
It leaves π as a symbol instead of rounding it. A radius of 5 gives an area of 25π, which is exact. Teachers often ask for this form. Multiply by 3.14159 to get a decimal.
Can the area be given in any unit?
Yes. The calculator does not convert units. Keep your input in a single unit, and read the circumference in that unit and the area in that unit squared.