Circle Calculator
Enter any one measurement — radius, diameter, circumference, or area — to instantly calculate all the others.
Radius
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Enter a value to calculate.
How the Circle formula works
The core circle formulas, all related through radius:
Diameter = 2 × radius Circumference = 2 × π × radius Area = π × radius²
Step-by-step calculation
- Convert whichever value you entered back into the radius (e.g., radius = diameter/2, or radius = √(area/π)).
- Use the radius to calculate the other three measurements with the standard formulas.
Worked example
A circle with radius 5: Diameter = 10. Circumference = 2π×5 ≈ 31.42. Area = π×25 ≈ 78.54.
Frequently asked questions
What's the difference between circumference and perimeter?
They describe the same idea — the distance around a shape — but 'circumference' specifically refers to circles, while 'perimeter' is the general term used for any shape.
Why is π (pi) used in circle formulas?
Pi is the fixed ratio between any circle's circumference and its diameter (about 3.14159), a mathematical constant that appears naturally any time circular measurements are related to each other.