Finding the size of a ball bearing in need of replacement, before purchase a new one, may save money. You also will have the correct replacement part the first time and avoid a return trip to the store. Determine ball bearing size by making a few measurements in millimeters. Ball bearings allow objects to move or rotate freely. Each disk shaped bearing contains a number of balls that roll. (See Reference 1) Ball bearings are used in industrial equipment as well as many types of wheels.