Circle calculator

Circle area = πr²; circumference = 2πr; diameter = 2r.

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Radius

Input mode

Answer

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Find radius from the selected measurement, then area = πr², circumference = 2πr and diameter = 2r. Radius = diameter/2, circumference/(2π) or √(area/π).

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Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

Save the current answer, then change an input to compare. This clears when you leave.

Formula and assumptions

Enter a radius in any length unit. Area uses that unit squared; circumference and diameter use the original length unit.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-26

Using Circle calculator

Find radius from the selected measurement, then area = πr², circumference = 2πr and diameter = 2r. Radius = diameter/2, circumference/(2π) or √(area/π).

Enter a radius in any length unit. Area uses that unit squared; circumference and diameter use the original length unit.

Next: Segment area calculator

A worked example

With radius = 3, the area is 28.274333882308138.

Next: Ellipse calculator

Before you use the result

Enter a radius in any length unit. Area uses that unit squared; circumference and diameter use the original length unit.

Example results for radius

These examples use Input mode: Radius. They are reference calculations, not recommended settings.

Radius	Area
1.5	7.06858347058
3	28.2743338823
6	113.097335529

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ToolOctopus. “Circle calculator.” Updated 2026-09-26. https://tooloctopus.com/circle-calculator.

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