Free · Circle calculators ⭕

Circle Radius from Circumference or Its Pi Coefficient

In full-circumference mode enter 28*pi. In π-coefficient mode enter 28. The coefficient field already supplies the π factor; adding another pi would multiply it twice.

Formula & worked example
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How to use this calculator

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.

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

Formula and method

Example in initial displayed units: Circumference (c) 87.96459 gives Radius (r) 14, Coefficient k in circumference kπ 28.

C = 2πr; if C = kπ then r = k/2

Worked example

Enter these known values and leave the other values blank.

Circumference (c): 87.9645943005142106769540147318260807575207431825029629872984485846188593760138519615849751095792779 cm

Radius (r): 14 cm

Assumptions and limitations

Ordinary mode accepts the full circumference, including an expression such as 28*pi. Pi-coefficient mode accepts k: enter 28 for a circumference of 28π. Entering 28*pi into the coefficient field would multiply by π twice. Optional diameter and area controls retain their visibility choices.
Use compatible units and enough independent values. Clear unknowns before solving backward. Original reference drawings are schematic and do not scale to entered values.

Common questions

What should I enter for a circumference of 28π?