WorldCalcs

Circle Calculator

Enter the radius to find a circle's diameter, circumference and area.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

Diameter

10

d = 2r

Circumference

31.4159

C = 2πr

Area

78.5398

A = πr²

What is a circle calculator?

A circle calculator works out a circle's key measurements from a single input, usually the radius. From the radius it finds the diameter (the distance across), the circumference (the distance around) and the area (the space inside). These come up constantly in geometry, design, engineering and everyday tasks like measuring round objects. This calculator gives all three at once.

How it's calculated

The radius r is the distance from the centre to the edge. The diameter is 2r. The circumference uses C = 2πr (or πd), and the area uses A = πr². The constant π is about 3.14159. If you know the diameter instead of the radius, halve it first.

Example

For a circle with a radius of 5, the diameter is 2 × 5 = 10, the circumference is 2 × π × 5 which is about 31.4159, and the area is π × 5² which is about 78.5398. A radius of 7 gives a diameter of 14, a circumference of about 43.9823 and an area of about 153.9380.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Type a positive radius. The calculator returns the diameter (2r), circumference (2πr) and area (πr²) instantly.

Values are rounded to about 6 significant digits using the standard constant π ≈ 3.14159.

Frequently asked questions

How do I find the circumference of a circle?+

Multiply the diameter by π, or use C = 2πr. For a radius of 5, the circumference is about 31.4159.

How do I find the area of a circle?+

Use A = πr², the radius squared times π. For a radius of 5, the area is about 78.5398.

What is the difference between radius and diameter?+

The radius is from the centre to the edge; the diameter goes all the way across and equals twice the radius.

What is pi?+

The ratio of a circle's circumference to its diameter, about 3.14159, the same for every circle.

How do I find the radius from the area?+

Divide the area by π and take the square root. See our Square Root Calculator.

How do I find the radius from the circumference?+

Divide the circumference by 2π.

What units does the area use?+

Square units of whatever you entered; if the radius is in cm, the area is in cm².

Can I enter the diameter instead?+

Halve the diameter to get the radius, then use these formulas.

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