Wrap a flexible measuring tape around the child's head. Start in the center of the forehead, 1 inch above the eyebrows. The measuring tape should be laying flat on the child's head, with no extra slack, and should not be pulled tightly.
Write down the measurement.
Select the size of the helmet that corresponds with the measurement taken. An extra small size should be selected for a head circumference of 20 3/4 inches to 21 1/4 inches; a small size should be selected for a circumference of 21 1/2 inches through 22 inches; select medium for head circumferences measuring 22 1/4 inches to 22 3/4 inches; a large helmet should be worn if the circumference measures 23 inches to 23 1/2 inches; an extra large helmet should be chosen if the head circumference falls between 23 3/4 inches and 24 1/4 inches.