Which value should I enter?

Enter any one of the four values — radius, diameter, circumference, or area. The calculator derives the other three automatically.

What is the formula for circumference?

Circumference = 2 × π × radius, or equivalently π × diameter.

Area = π × radius². For example, a circle with radius 5 has area π × 25 ≈ 78.54 square units.

The calculator uses JavaScript's Math.PI constant, which is approximately 3.141592653589793.

Use any consistent unit. If you enter the radius in centimetres, the circumference will be in centimetres and the area in square centimetres.

Calculate the radius, diameter, circumference, and area of a circle. Enter any one value and the remaining three are calculated automatically.

Radius (r)

Diameter (d)

Circumference (C)

Area (A)

Learn More

Understanding Area Measurements and Area Calculations

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Step 1: Enter a value in any one of the four fields — radius, diameter, circumference, or area.

Step 2: The remaining values are calculated instantly and displayed below.

Enter only one value at a time. Use the Reset button to clear all fields and start over.

A circle is defined by its radius (r). All other properties can be derived from it.

Radius (r): Distance from the centre to the edge.

Diameter (d): d = 2 × r

Circumference (C): C = 2 × π × r = π × d

Area (A): A = π × r²

π (pi) ≈ 3.14159. The calculator uses JavaScript's Math.PI for maximum precision.

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