Radius, diameter, area & circumference

Circle Area & Circumference

Enter any one circle measurement and instantly see the radius, diameter, area and circumference.

Value

Radius—

Diameter—

Area—

Circumference—

How it works

1

Enter a value

Type any positive number — a radius, diameter, area or circumference reading.

2

Pick its type

Choose which measurement you entered so the tool knows how to solve for the rest.

3

Read every measurement

All four values update instantly using the exact value of pi.

FAQ

What is the formula for circle area?

The area of a circle is π multiplied by the radius squared: A = πr². Given the diameter d instead, radius is d ÷ 2.

What is the formula for circumference?

Circumference is 2π multiplied by the radius, or equivalently π multiplied by the diameter: C = 2πr = πd.

Can I enter the area or circumference instead of the radius?

Yes. Choose "Area" or "Circumference" as the input type and the tool solves backward for the radius, diameter and the other measurement.

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