2. Determine the Proper Helmet Shape: Heads vary in shape. Heads that seem to be the same size when measured by a tape may not necessarily fit the same size motorcycle helmet. Most helmet manufacturers today offer a number of different shell shapes; typically oval/narrow (example, Arai Profile, Shoei X11), neutral/intermediate (Nexx Carbon, Shoei X-12) and round (Arai Quantum-2). You can determine your head shape by looking in the mirror. If your head is long from top to bottom and narrow from side to side, you are going to fit best in an oval shape. If you're not sure, try an intermediate shape.