Circle Calculator
Enter the one measurement you know. The calculator works out the other three properties of the circle.
Result
- Radius
- 5 cm
Formula
- Diameter d = 2r
- Circumference C = 2πr = πd
- Area A = πr²
- r = C ÷ (2π) = √(A ÷ π)
How the Circle Calculator works
Every circle is defined by one number. The radius is the distance from the center to the edge and the diameter is twice that. The circumference is the distance around, which is always π times the diameter.
π is the ratio of circumference to diameter and is about 3.14159. It is irrational, so its decimal expansion never ends or repeats.
Worked example
A circle with a radius of 5 cm has a diameter of 10 cm, a circumference of 2 × π × 5 = 31.4159 cm and an area of π × 5² = 78.5398 cm².
Frequently asked questions
How do I find the diameter from the circumference?
Divide the circumference by π. A circumference of 100 gives a diameter of 100 ÷ 3.14159 = 31.83.
How do I find the area from the diameter?
Halve the diameter to get the radius, square it and multiply by π. Or use A = π × d² ÷ 4.
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