Circle Calculator (Radius • Diameter • Circumference • Area)

Relations: C = 2πr, d = 2r, A = πr², and inverses r = d/2 = C/(2π) = √(A/π). All fields sync.

Radius (r)

Value

Tip: Choose mixed units (ft/in or m/cm) if that’s easier to enter.

Diameter (d)

Value

d = 2r

Circumference (C)

Value

C = 2πr

Area (A)

Value

A = πr²

Step-by-step derivation

Enter any one field to get started…

References & Notes

Circle relations: C = 2πr, d = 2r, A = πr². Inverses: r = d/2 = C/(2π) = √(A/π).
Mixed units are supported for convenience (ft/in and m/cm).
Displayed values keep up to two decimals and update in real time.

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