Parallel Axis Theorem

Let $I_0$ denote the moment of inertia for a rotation axis passing through the center of mass, and let $I_d$ denote the moment of inertia for a rotation axis parallel to the first but a distance $d$ away from it. Then the parallel axis theorem says that

$$I_d = I_0 + Md^2$$

where $M$ denotes the total mass. Thus, the added inertia due to displacement by $d$ meters away from the centroidal axis is equal to that of a point mass $M$ rotating a distance $d$ from the center of rotation.