Enter a radius or diameter to get the area, circumference and diameter of a circle.
What this calculator does
This circle calculator finds every basic property from one measurement. Enter the radius or the diameter and it returns the area, circumference and the other dimensions.
Handy for geometry homework, DIY and design.
Worked example
The calculator opens with a radius of 5. On those figures the area is 78.54. Underneath, Diameter comes out at 10 and Circumference at 31.416. Change any field and every figure updates as you type.
How to fill it in
- Choose whether you know the radius or the diameter.
- Enter that value.
- Read the area, circumference and diameter.
How the number is calculated
area = π × r²
circumference = 2 × π × r · diameter = 2 × r
Common questions
How do I find the area of a circle?
Multiply pi by the radius squared (πr²). With a diameter, halve it first to get the radius.
What’s the circumference formula?
2πr, or π times the diameter. It’s the distance around the circle.
What is pi?
A constant, about 3.14159, that relates a circle’s circumference to its diameter.