Circle calculator
Know one measurement of a circle, get the other three.
Area
Rounded for displayArea: ≈ 78.5398
- Radius
- 5
- Diameter
- 10
- Circumference
- ≈ 31.4159
How it works
Everything about a circle follows from its radius r. The diameter is twice the radius, the circumference is 2πr and the area is πr².
From the circumference, r = C ÷ 2π; from the area, r = √(A ÷ π). Results are in the same unit as your input, squared for the area.
Formula
d = 2r C = 2πr = πd A = πr² r = √(A ÷ π)
Worked example
A circle with radius 5 has diameter 10, circumference 2π × 5 ≈ 31.41592654 and area π × 5² ≈ 78.53981634.
Questions people ask
What is π?
π is the ratio of any circle’s circumference to its diameter, about 3.14159265. It is irrational, so its decimals never end.
How do I find the radius from the area?
Divide the area by π and take the square root: an area of 50 gives r = √(50 ÷ π) ≈ 3.989.
What units are the answers in?
The same unit you used: centimetres in give centimetres for lengths and square centimetres for the area.