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Math

Circle Area Calculator

Calculate circle area, circumference and diameter from its radius.

Exact method explained belowEditable inputs
Geometry desk
Geometry desk

Match each dimension to the shape and keep units consistent.

See the method

Set the dimensions

Runs on your device

Your measurement

Your result

Let’s calculate.

Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter radius.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

Input reference and sample values

Use the units and choices shown for each field. Sample values are provided for practice, not as recommendations.

InputFormat and unitsSample
RadiusNumeric value. Zero or positive.5

Result for these sample inputs

78.5398163 square units
Circumference
31.4159265
Diameter
10

Area is in square units. Circumference and diameter use your radius unit.

Select Try the example in the form to load these values and calculate them again. This replaces the current form inputs. Compare the result with the method and worked explanation below.

The method, explained

Area = πr². Circumference = 2πr. Diameter = 2r. Keep the same length unit throughout the calculation.

A WORKED EXAMPLE

A radius of 5 gives an area of about 78.54 square units, circumference of 31.42 units and diameter of 10 units.

What to keep in mind

Enter radius, not diameter. If your radius is in centimeters, the area is in square centimeters. Results use the available numerical approximation of π.

Common questions

What if I know the diameter?

Divide the diameter by two, then enter that value as the radius.

Does changing length units also change area units?

Yes. An area conversion squares the length conversion factor. Use Area Converter to convert the resulting square units.

Methodology maintained by ClarityKit. How these tools are built and checked.