Circle area calculator

Compute circle area from radius or diameter, with unit conversions and configurable decimals.

Circle area calculator

Work out the area of a circle online from a radius, diameter or known surface value in seconds.

Review the main area alongside the equivalent radius and diameter plus automatic conversions to mm², cm², m², in² and ft² with your preferred decimal precision.

Input data

Method

Radius Diameter Circumference Area

Radius e.g. 7.5

Accepts comma or dot as decimal separator.

Units

The main area is displayed in unit².

Decimals 0–10

Applies rounding to every result.

Value of π

Provide your own π if you work with a specific approximation.

Results

Main area

Enter your data and press Calculate

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Radius (r)

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Diameter (d)

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Circumference (C)

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Area (A = π·r²)

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Area conversions

mm²

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cm²

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m²

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in²

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ft²

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Includes every output with its unit.

Formulas used

Area from radius: A = π · r²
Area from diameter: A = π · (d/2)²
Area from C (edge length): A = C² / (4 · π)
Relationships: d = 2r and C = 2πr

How do I calculate a circle’s area with this tool?

Select whether you know the radius, diameter, circumference or an existing area, enter the value in the correct unit and press Calculate. The results card shows the main surface plus the matching radius, diameter and perimeter.

Can I enter the circumference and get radius and diameter?

Yes. Pick the “Circumference” mode, type the edge length and the calculator solves for radius and diameter before returning the area, so you don’t need to divide by 2π on your own.

Which units are supported and how are conversions handled?

You can work in metres, centimetres, millimetres, inches or feet. After computing, the area is shown in the entered unit and also in mm², cm², m², in² and ft², and you can copy every output with one click.

How precise are the results?

You can set 0–10 decimals and choose π between Math.PI, the 22/7 approximation or your own custom value. Rounding is applied consistently to every displayed measurement.

Which formulas does the calculator use?

It relies on A = π·r² for the area, d = 2r for the diameter and C = 2πr for the circumference. When you enter C it applies A = C² / (4·π), then converts each result to the listed units.