Free · Circle calculators ⭕

Circle Boundary Length and Enclosed Area

Circumference measures boundary length and uses length units. Enclosed area measures the disk and uses squared length units. Both can determine the positive radius of the same circle.

Formula & worked example
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Calculator inputs

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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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Formula and method

Example in initial displayed units: Area (A) 78.53982 gives Radius (r) 5, Diameter (d) 10, Circumference (c) 31.41593.

C = 2πr; A = πr²; r = √(A/π)

Worked example

Enter these known values and leave the other values blank.

Area (A): 78.5398163397448309615660845819875721049292349843776455243736148076954101571552249657008706335529267 cm²

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

Assumptions and limitations

Boundary length and enclosed area are different quantities. Enter one positive circle measure to derive the others, with compatible length and area units. The area inverse returns the positive radius.
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

How do circumference and enclosed area differ?