A ship's mass radii of inertia strongly influence its seakeeping performance. For example, the pitch radius of inertia (kyy) affects pitch motions and therefore also vertical accelerations, deck wetness and added propulsive resistance in waves. The roll radius (kxx) and yaw radius (kzz) of inertia have a similar influence on roll, yaw and transverse accelerations, affecting comfort and the loads acting on cargo. To predict the seakeeping performance (numerically or experimentally) of a ship during design requires an understanding as well as prediction of the radii of inertia. These can only be calculated late in the design process once a detailed knowledge of the distribution of weight through the ship is available. Earlier in the process the designer must rely on estimation techniques. A number of estimation approaches are available, normally based on assuming the radius of inertia is some fixed fraction of the length or beam. Grin and Fernandez (2015) demonstrated that small errors in this estimate can lead to large errors in predicted motions, and proposed an estimation method which gave more accurate results for a small range of example ships.