Let IG be the moment of inertia of a rigid body about an axis through G, the centre of mass of the rigid body. Then the moment of inertia of the body about some other axis parallel to the first axis equals IG + md2, where m is the mass of the rigid body and d is the distance between the two parallel axes.
Rephrasing the theorem in terms of the inertia matrix, for a point P at (column) position vector r from G, then
where I3 denotes the 3×3 identity matrix.