Circle Calculator
Area and circumference of a circle from its radius.
How it works
Area = πr²; Circumference = 2πr
π ≈ 3.14159265… Area grows with the square of the radius, so doubling r quadruples the area.
FAQ
What about the diameter?
Radius = diameter ÷ 2. Convert the diameter to radius first.
Worked example
Radius 5 cm:
- Circumference
2πr = 31.42 cm - Area
πr² = 78.54 cm²
A 10 cm diameter gives the same result — the radius is half of it.
Notes and limitations
The classic error is plugging the diameter in as the radius. Area grows with the square of radius: double the radius, quadruple the area, double the circumference.