Free · Plane geometry calculators 🔺

Circle Boundary Length from Radius or Diameter

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

  • Formula & worked example
  • Private in your browser
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Calculator inputs

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How to use this calculator

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

Private by default

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

Formula and method

For radius 5 in the selected length unit, diameter is 10; boundary length is 10π and area is 25π in compatible units.

d = 2r; C = 2πr

Worked example

Enter these known values and leave the other values blank.

Radius (r)
5 cm
Diameter (d)
10 cm
Circumference (c)
31.416 cm

Assumptions and limitations

  • 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.

Common questions

How do radius and diameter relate?

Diameter is twice the radius. Enter a known length and clear the quantity you want to infer; the available controls retain their captured unit selectors.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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