Circle Radius Calculator

Work out circle radius instantly with clear inputs, formula shown and shareable results.

About Circle Radius Calculator

The radius can be recovered from any one circle measurement: r = √(A/π) from area, r = C/(2π) from circumference, or r = d/2 from diameter.

Formulas

From area

r = √(A / π)

From circumference

r = C / (2π)

Worked example

With the default values below, Circle Radius Calculator returns Radius of 5.0000. Change any input above and every figure here is recalculated in your browser.

Inputs used

What you knowone of Area, Circumference, Diameter: Area
Its valueminimum 0: 78.539816

Results

Radius: 5.0000
Diameter: 10.0000
Circumference: 31.4159
Area: 78.5398

Step by step

1Values used
2From area
3From circumference
4Radius= 5.0000
5Diameter= 10.0000
6Circumference= 31.4159
7Area= 78.5398

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Area 78.539816 gives what radius?

√(78.539816 / π) = 5 exactly.

Which input is most accurate in practice?

Circumference, because a tape around the object avoids having to locate the centre.