Free · Plane geometry calculators 🔺

Circle Radius Recovered from Boundary Length

Calculate circle radius recovered from boundary length with bidirectional inputs, compatible units and clear geometric assumptions.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Boundary length 10π centimeters gives radius 5 centimeters.

d = 2r; C = 2πr; A = πr²

Enter these known values and leave the other values blank.

Circumference (c): 31.4159265358979323846264338327950288419716939937510582097494459230781640628620899862803483 cm

The model describes a Euclidean circle. Radius is half the diameter; boundary length and enclosed area have different physical dimensions.
Length and area selectors are converted before calculation. A squared-length unit is not the same as a length unit.
The supplied measurements must be mutually consistent. Clear an unknown value rather than supplying conflicting dimensions.
Arithmetic uses 64 significant digits and full-precision π where applicable; displayed values are rounded separately.

Use an area input in a squared-length unit and clear the length you want to infer. The positive geometric root is used.

The international acre is exactly 43560 square international feet; Bookify replaces the rounded captured area factor.