How to use the Circle Calculator
- Choose which measurement you know.
- Enter its value.
- Read the radius, diameter, circumference and area.
What does this tool do?
Every circle is defined by its radius. The diameter is twice the radius, the circumference is 2πr and the area is πr². Knowing any one of these is enough to work out the rest.
π (pi) is about 3.14159; the calculator uses it to full double precision.
Why use it?
- Start from any measurement.
- All four values at once.
- Full-precision π.
The formula
d = 2r
C = 2πr = πd
A = πr²
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Last reviewed by the M2Toolkit team.