The tailor's measuring tape is the greatest tool for measuring the head. You may also wrap a non-elastic thread around your head as an alternate tool. Two finger widths directly above the brows and above the ears, as well as the most broad region at the backside of the head, are excellent spots to measure for dirt bike helmet size. The DOT (Department of Transportation) standard for dirt bike helmets is available here: http://www.dot.gov/safety/helmets/index.htm. It is important to note that not all dirt bike helmets comply with this standard. Some companies may choose to produce their own sizing system by measuring several different locations on the head and then averaging these numbers. This is especially common with small or medium sized heads. For example, one company may choose to measure along the sides of the head under the ear canals and then another measurement a few inches farther back from each side point. These two numbers are then averaged to come up with a final size. Dirt bike helmets should be large enough to provide adequate coverage for low speed impacts while still allowing for sufficient ventilation. Most manufacturers claim that their products will protect you from serious injury in a high speed impact. However, you should always wear a properly fitted helmet when riding because no matter what type of protection you get from your helmet, it cannot replace the importance of safe riding practices.