Circle calculator
Know one measurement of a circle? Get the other three.
I know the
Radius r5
Diameter d = 2r10
Circumference C = 2πr31.41592654
Area A = πr²78.53981634
Where π comes from: unroll the circle
Roll a circle of diameter 1 along a line for one full turn. The distance it travels is its circumference, and that distance is always π ≈ 3.14159 diameters, whatever the size of the circle. Drag the slider to roll it.
0.00000 diameters
Keep rolling…
Circle formulas
- d = 2r
- C = 2πr = πd
- A = πr² = πd²/4
- r = C/(2π) = √(A/π)
For arcs and pie slices, use the circle sector option in the area calculator.
Questions people ask
How do I find the radius from the circumference?
Divide by 2π: r = C ÷ (2π). For a circumference of 31.4 cm, r ≈ 5 cm. Choose “Circumference” as the known value above.
How do I find the radius from the area?
r = √(A ÷ π). An area of 78.54 cm² gives r ≈ 5 cm.
What value of π does the calculator use?
JavaScript’s Math.PI, 3.141592653589793, accurate to about 16 significant digits. Using 3.14 instead introduces an error of about 0.05%.
What is the difference between circumference and perimeter?
None in meaning: circumference is the name for the perimeter of a circle.