What are the circle formulas?

Diameter = 2r. Circumference = 2πr = πd. Area = πr². All derive from π, the ratio of circumference to diameter.

How do I find the radius from the area?

r = √(A / π). From circumference: r = C / (2π). From diameter: r = d / 2.

What is π used here?

JavaScript's Math.PI — double-precision (≈3.141592653589793). Accurate to ~15 significant digits, well beyond most practical needs.

Does this handle spheres?

Yes. The result panel also shows sphere surface area (4πr²) and volume (4/3 · πr³) derived from the same radius.

Circle Calculator

Calculators· 14 tools

Free circle calculator. Enter any one of radius, diameter, circumference, or area, and get the other three instantly. Bonus: sphere surface area and volume from the same radius. Uses double-precision π.

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Radius (r) value

Units are consistent — if you enter a radius in cm, area comes out in cm². Volume in cm³.

Circle: C = 2πr, A = πr²

Sphere: S = 4πr², V = (4/3)πr³

Radius (r)

Diameter (d)

Circumference (C)

31.4159

Area (A)

78.5398

Sphere (same radius)

Surface area314.1593

Volume523.5988

Frequently Asked Questions

References

Circle — Weisstein, Eric W. MathWorld — Wolfram MathWorld

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References and sources

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