How is circle area calculated?

π × radius². E.g. radius 10 cm → π × 100 ≈ 314 cm².

What is the circumference?

2 × π × radius, i.e. π × diameter.

Circle Calculator

Calculate a circle's area, circumference and diameter from its radius.

Radius: 10.00 cm

314.16 cm²

Area

Area	314.16 cm²
Circumference	62.83 cm
Diameter	20.00 cm

ℹ️ Area = π × radius². Circumference = 2 × π × radius. Diameter = 2 × radius.

The circle calculator finds a circle's area, circumference and diameter from its radius. It is useful for schoolwork, crafts and building tasks where you need to size round shapes quickly.

How the calculator works and what it’s for▾

The formulas used

The area is pi times the radius squared. The circumference is two times pi times the radius, and the diameter is simply two times the radius.

For example, with a radius of 5 cm the area is about 78.5 square centimetres, the circumference about 31.4 centimetres and the diameter 10 centimetres.

What you enter and what you get

You only need to provide the radius in centimetres. From that the calculator returns the area, the circumference and the diameter all at once.

If you only know the diameter, divide it by two before entering it to get the radius.

What it is for

The calculator helps with maths exercises, but also in practice: estimating the area of a round tabletop, the size of a circular rug or the distance around a pipe.

Good to know

Pi is an infinite decimal, so results are always slightly rounded. For most everyday purposes a couple of decimal places is more than accurate enough.

🔄 Reviewed June 2026