Mass Moments of Inertia (MOI) are important and often critical components of the mass properties of a vehicle, but many Mass Properties Engineers tend to focus only on weight and center of gravity (CG), and have limited exposure to these other, rotational properties. In this paper, I present a brief overview of MOI, why they are important, and a method for quickly estimating the MOI of a part, subassembly, or assembly. This method is particularly useful when reviewing CAD calculations or a supplier’s mass properties reports in which you don’t have visibility into the details to ensure that the results are reasonable. You can also use this technique to make a quick MOI estimate for trade studies. While there are some limitations to this method which are described in this paper, this technique will get you in the ballpark and increase your confidence that the rotational properties of your vehicle are properly represented.