Circle Calculator
Calculate the circumference, area, diameter, or radius of a circle from any one known measurement.
Inputs
Radius
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Diameter
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Circumference
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Area
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About Circle Calculator
Formulas
d = 2r. c = 2πr = πd. A = πr². Memorize these four and you can derive any circle property from any other.
Pi (π)
The ratio of a circle’s circumference to its diameter. Approximately 3.14159265... — an irrational number with no exact decimal representation. This calculator uses JavaScript’s Math.PI (~15 decimal places).
Common uses
Pizza math (compare areas: 10" vs 12" pizza). Bicycle wheel rotations from circumference. Cake pan sizing. Garden bed area. Wire length wrapped around a cylinder.