A circle does not have a volume. The volume of an object is the measurement of how much an object holds. It is a cubic measurement and does not apply to two-dimensional objects such as circles.
The formula for determining volume varies with the shape of the object. A cylinder that has a circular base has volume because it is three dimensional. The formula for determining its volume is pi times the radius of the circular base squared times the height of the cylinder. The area of a circle is found by the formula pi times the radius squared. Pi is equal to approximately 3.14, and the radius is half of the circle's diameter.