Bicycle helmets are designed to help prevent head injuries and death. A bike helmet for the average person is fitted usually by measuring the circumference of the head and then matching that measurement with the size chart on the helmet. Since heads are shaped differently, measure the helmet yourself in several places, compare with your own head measurements and try the helmets on to help you find the right helmet for your head.
Use a flexible, soft measuring tape designed to take body or tailoring measurements. A metal, stiff measuring tape will not yield accurate results.
Measure the circumference of your head, right above the eyebrows or ears. Then, measure from the front of your head, around the side, to the back of your head and then from one side of the head to the other (just above the ears). Start with the end that reads zero. The tape measure in the back should be even with the portion that is in the front. If possible, have a second person help you.
Measure the helmet's inside circumference, just at the edge or rim. Then, measure from the front of the helmet to the back and then side to side. Compare the results with your head measurements. The bicycle measurement should be larger than your head measurement, up about 3.8 to 5 cm (1 1/2 to 2 inches) before you have to move down the next size. If the helmet is the same size or smaller than your head, measure the next larger sized helmet. Try a different brand helmet for a different shape to better match your head.
Try on the helmet to get a good match. It should fit your head shape and not be too loose. Remember, you still have fitting pads to install. If they are already in place, the helmet should fit snug and comfortable on your head. It should not move as you move around and you should not easily be able to move it around with your hands. The goal is for the helmet to stay just where it is if you get in an accident, but at the same time, be comfortable.