To find the correct motorcycle helmet size, measure the head at its largest circumference, which is above the eyebrows, over the eyes and around the back of the head. Helmets are sold in sizes of S, M, L and XL, and every manufacturer has its own size equivalents.
For the best fit, try on the helmet. The helmet should fit snugly and squarely on the head. The cheek pads should touch the cheeks, and there should be no gaps between the side of the head at the temples and the brow pads. When the helmet is removed, there should be no sore or red spots on the head, according to Helmet Check.