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Circle Calculator

Calculate circumference, area, diameter, or radius of a circle from any one known measurement.

Calculator

Enter your values and view the result instantly.

Change any field below to update the answer straight away.

Known measurement

Unit

78.5398 cm²

Area

31.4159 cm

Circumference

10 cm

Diameter

5 cm

Radius

Geometry Basics

Circle area, circumference, diameter, and radius

A circle calculator helps you solve the whole shape from one known measurement. Enter radius, diameter, circumference, or area and the calculator works out the rest, making it a useful geometry calculator and conversion calculator for school, construction, and everyday maths.

How circle measurements connect

Every circle is built around the same relationships. The radius is the distance from the centre to the edge. The diameter is twice the radius. The circumference is the distance around the circle, and the area is the amount of flat space inside it.

Because all of these measurements are linked, one known value is enough to calculate the others. That is why a circle calculator is such a practical online maths tool. Whether you start with a radius, a diameter, a circumference, or an area, the rest follow from the same geometry.

Core circle formulas

Circle formulas all depend on pi, written as π. Pi is the constant ratio between circumference and diameter and is approximately 3.14159.

Diameter = 2r

If the radius r is known, the diameter is simply twice that value.

Circumference = 2πr = πd

The distance around the circle can be found from either radius r or diameter d.

Area = πr²

The area inside the circle depends on the square of the radius, which is why area grows quickly as the circle gets larger.

Why this calculator is useful

A free online calculator for circles is useful in more places than a classroom. It can help with round tables, pipes, circular slabs, wheels, lids, tanks, sports markings, and any layout where a radius or diameter is known but area or perimeter is needed.

It is also a helpful problem-solving calculator because it lets you switch starting points. If you only know circumference, the tool can work backward to radius. If you only know area, it can estimate the diameter. That makes it a practical everyday maths calculator rather than just a formula lookup.

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