Before purchasing a motorcycle helmet, a rider should measure his head at the largest circumference, which is typically just above the eyebrows. Since sizes of helmets typically range between small and extra-large, check with the manufacturer to determine size equivalents.
Since most people know their hat size, that is a good place to start when looking at motorcycle helmets. Once a rider knows the size of his head, trying on various helmets is the best way to determine a proper fit.
When trying on a helmet, slip it on using the chin straps. The helmet should sit squarely on the head and fit snugly. The cheek pads should touch the cheeks but not press down in an uncomfortable way. Make sure there are no gaps between the brow pads and temples and the neck roll should not be large enough to push the helmet away from the head. A helmet that fits properly will not move when moved with the hands.
Moving the helmet should be a struggle, and the helmet should be snug yet comfortable. If the helmet can be rolled off the head when using a side-to-side motion, it is too large. Be sure the helmet is not pressing down on pressure points or it may cause a headache.
A helmet that is too small will be uncomfortable, and discourage the rider from wearing it. A helmet that is too large might come off in an accident, resulting in serious injury or death.