The equation
gives the moment of inertia of a rectangle about its own centroidal axis (axis c in the sketch).
Parallel-axis theorem
If the moment of inertia Ic for an area is known about a centroidal axis, the moment of inertia I of the area about a corresponding parallel axis can be computed from:
where d = perpendicular distance between the centroidal axis and any parallel axis and A = area of shape.
