The formula for moment of inertia for a circle is the product of pi over four times the radius to the power of four. The area moment of inertia is also called the second moment of area.

In mathematical notation, the moment of inertia is often symbolized by I, and the radius is symbolized by r. The formula for the moment of inertia of a ring created by two overlapping circles is similar. Multiply pi over four by the difference between both radii taken to the fourth power. The moment of inertia of part of a circle can be found by using the trigonometric function sine and the angle of the circle segment in question.