How it works
A circle has only one true degree of freedom — its radius. Diameter and circumference are both fixed multiples of that radius, so knowing any one of the three tells you the other two immediately, without needing a second measurement.
diameter = 2 × radius
circumference = 2 × π × radius = π × diameter
Worked example
A circle with radius 5.
- Diameter: 2 × 5 = 10.
- Circumference: 2 × π × 5 = 31.42.
The circumference is 31.42 (diameter 10).
Common questions
What is π (pi)?
Pi is the fixed ratio between a circle's circumference and its diameter — roughly 3.14159, and the same for every circle regardless of size. It's an irrational number, so this calculator uses your browser's built-in double-precision value rather than a rounded approximation.
Circumference vs. area — what's the difference?
Circumference is the distance around the circle (a length); area is the space it encloses (a squared measurement). If you need area instead, see the Area of a Circle Calculator, which accepts the same three starting values.
Do I need to know the units in advance?
No — enter whatever unit you're measuring in (inches, centimetres, metres, anything), and the circumference comes back in that same unit. The relationship between radius, diameter and circumference doesn't depend on which unit you use.