Parallel Axis Theorem

For an area $A$ whose centroidal axis is displaced $d$ from the axis of rotation along $x$, the moment of inertia about the $x$ axis is given by

$$I_x(d) = I_x(0) + Md^2$$

where $M$ denotes the total rotating mass, and $I_x(0)$ is defined as the moment of inertia of area $A$ about its centroidal axis parallel to the $x$ axis. Thus, the added inertia due to displacement by $d$ meters from the centroidal axis is equal to that of a point mass $M$ rotating a distance $d$ from the center of rotation.