How To Measure Your Head Size 1. To determine where you fall on Crown's sizing chart (as a starting point for proper helmet fit), pass a soft, flexible tape measure horizontally around your head at a height of approximately one inch (2.5cm) above your eyebrows (the largest measurement around your forehead). 2. Select the helmet size that is closest to your head measurement from the sizing guidelines above. If your head measurement falls between two helmet sizes, first try on the smaller of the two helmets.