Circle Area Calculator

Calculate the area of a circle from its radius or diameter. Also shows circumference and diameter.

Input

Enter a radius or diameter to compute the circle area, circumference, and diameter.

Radius r

Area

78.539816

Circumference

31.415927

Diameter

Lengths share the input unit, and the area uses that unit squared.

The area is computed as A = π × r², where r is the radius.
When you enter a diameter d, the radius is taken as r = d ÷ 2.
The circumference is C = 2 × π × r and the diameter is d = 2 × r.
Pi is treated as approximately 3.141592653589793.
Lengths use the same unit as your input, and the area uses that unit squared. For example, entering cm gives an area in cm².

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