Free · Plane geometry calculators 🔺

Circumference with Radius and Diameter Controls

Calculate circumference with radius and diameter controls with bidirectional inputs, compatible units and clear geometric assumptions.

  • Formula & worked example
  • Private in your browser
  • No signup
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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; A = πr²

Worked example

Enter these known values and leave the other values blank.

Radius (r)
5 cm
Diameter
10 cm
Circumference (c)
31.4159 cm
Area
78.5398 cm²

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

Can I enter an area to recover a radius or diameter?

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

References

Bookify requires nonnegative geometric measurements rather than permitting signed lengths or enclosed areas. The international acre is exactly 43560 square international feet; Bookify replaces the rounded captured area factor.

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