T' diameter o' a hobby rocketry motor casing. Most commercially-manufactured motors are made in standard sizes. Well, blow me down! Motor diameters are most commonly expressed in millimeters. Avast, me proud beauty! T' most commonly seen motor diameters are 10.5mm, shiver me timbers, 13mm, 18mm, arrr, 24mm, 29mm, 38mm, 54mm, 75mm, and 98mm.