DocomintCircle Calculator
Calculate a circle's radius, diameter, circumference, and area from any one known value.
Most circle calculators assume you're starting from the radius, but real problems often hand you the circumference (a tape measure around a pipe) or the area (a spec sheet) instead — this works backward from any of the four values to fill in the other three.
Example: A circular table with a 113.1 in² tabletop (area known) works out to a radius of about 6 inches, a 12-inch diameter, and a circumference of about 37.7 inches — handy for figuring out the diameter without being able to measure straight across the middle.
How it works
- Choose which value you already know — radius, diameter, circumference, or area
- Enter that value
- The other three values appear instantly, computed on your device
Frequently asked questions
How do I find a circle's area if I only know the circumference?
First solve for radius: r = circumference / (2π). Then compute area = π × r². Select "Circumference" as the known value here and it handles both steps for you.
What's the difference between radius and diameter?
Radius is the distance from the center of a circle to its edge. Diameter is the distance all the way across, through the center — always exactly twice the radius.
Is my numbers uploaded anywhere?
No — Circle Calculator runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.