Free · Circle calculators ⭕

Circle Radius, Diameter, Circumference and Area

Enter a positive radius, diameter, circumference or area. The other values describe the same circle. Area-to-radius solving uses the positive square root, and length and area use different unit dimensions.

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.

Private by default

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: Radius (r) 3 gives Diameter (d) 6, Circumference (c) 18.84956, Area (A) 28.27433.

d = 2r; C = 2πr; A = πr²

Worked example

Enter these known values and leave the other values blank.

Radius (r): 3 cm

Diameter (d): 6 cm
Circumference (c): 18.8496 cm
Area (A): 28.2743 cm²

Assumptions and limitations

All measures describe one circle. Length and area selectors convert their respective dimensions. Radius and area inverses use positive roots, and this workflow requires strictly positive measures.
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

Which circle measurement can I enter first?

References

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

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