Circle Calculator
Know one measurement of a circle? Enter it to get the radius, diameter, circumference and area.
Result
Enter values and press Calculate to see the result.
Formula
Circumference is the distance around the circle.
Area is the space inside the circle.
How to use this calculator
- Choose which measurement you know and enter it.
- Press Calculate. Exact answers are given in terms of π, with decimals underneath.
Worked example
A circle with radius 5
Diameter = 2 × 5 = 10.
Circumference = 2π × 5 = 10π ≈ 31.415927. Area = π × 5² = 25π ≈ 78.539816.
Frequently asked questions
Why is the answer written with π?
π is irrational, so “25π” is the exact answer. The decimal below it is rounded.
What units should I use?
Any unit works. Lengths come out in the same unit; area comes out in that unit squared.