Circle calculator
Start from any one measurement of a circle and get the rest. It is handy for geometry homework, garden and craft projects, and sizing round tables, pipes or pans.
- Runs in your browser
- Free, no account needed
- Copy or download the result
Result
Example
I know the: Radius Value: 5 Decimal places (0 to 12): 4
Radius: 5 Diameter: 10 Circumference: 31.4159 (10π) Area: 78.5398 (25π) Formulas: diameter = 2r, circumference = 2πr, area = πr².
How to use the circle calculator
- Under I know the, choose Radius, Diameter, Circumference or Area.
- Type the Value and set Decimal places (0 to 12).
- Press Run to see the radius, diameter, circumference and area.
- Press Copy result or Download to keep the results.
How it works
The formulas are d = 2r, C = 2πr and A = πr². Starting from area, the radius is √(A / π).
When you start from the radius or diameter, the circumference and area also show an exact multiple of π, such as 10π.
Results are rounded to the decimal places you choose, and values too small for those places are shown with significant digits instead of 0.
The value must be greater than zero. Area uses the square of whatever length unit you use.
Everything runs in your browser. Your input is not uploaded or stored.
Questions and answers
How do I find the area of a circle?
Multiply π by the radius squared. A circle with radius 5 has area 25π, about 78.54.
How do I find the radius from the circumference?
Divide the circumference by 2π. A circumference of 31.4159 gives a radius of about 5.
What is the difference between radius and diameter?
The radius goes from the center to the edge. The diameter goes all the way across, so it is twice the radius.