How do I find the area of a circle?

The area is pi times the radius squared. So a circle with a 5 unit radius has an area of pi times 25, about 78.5 square units. Enter the radius and the calculator gives the area, circumference, and diameter.

What is the circumference of a circle?

The circumference is the distance around the circle. It is pi times the diameter, or two times pi times the radius. For a 5 unit radius it is about 31.4 units.

What is the relationship between radius and diameter?

The diameter is twice the radius, the full distance across the circle through the centre. The radius is half the diameter. The calculator converts between them automatically.

Circle Calculator

Work out everything about a circle from one measurement. Enter the radius and the calculator gives the diameter, the circumference, and the area straight away. Useful for geometry homework, garden beds, pipes, and round tables.

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Diameter0

Circumference0

Area0

The formulas

Diameter = 2 times the radius
Circumference = 2 times pi times the radius
Area = pi times the radius squared

Worked example

A circle with a radius of 5 has a diameter of 10, a circumference of about 31.42, and an area of about 78.54. The units follow whatever you used for the radius, with area in square units.

Related calculators

Cylinder Volume Calculator: volume of a cylinder.
Sphere Volume Calculator: volume of a sphere.
Square Metre Calculator: work out an area.
Trigonometry Calculator: angles and sides.

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Reviewed and maintained. Last reviewed 2026-06-07 and checked on a twice-monthly cycle against IRD, RBNZ and Stats NZ. How we keep data current.