Home Advisor How to choose a helmet size - proper fit of the helmet How to choose a helmet size - proper fit of the helmet Measure your head circumference in centimeters approx. 1-2 cm above the eyebrows. Divide the measured value in "cm" by a factor of 2.54 and find the obtained value in "inches" in the table of the appropriate helmet sizes. If you are between two sizes, choose a smaller rather than a larger helmet size. The helmet must be firmly attached and close to the head. If your helmet is too loose, you have the wrong size. Instructions video: Share Tweet Pin It