Proper helmet fit is vital to the performance of a helmet during an impact. For the helmet to perform properly during an impact, it must stay securely on the head.
Measuring the head is only a starting poing for the entire sizing procedure. Due to varying shapes, heads that are apparently teh same size when measured by a tape may not necessarily fit the same size helmet.
A small metal tape measure or cloth may be used to make your inital measurment. You can also use a string, which can then be laid against a measuring tape.
Measure the circumference of the head at a point approximatley one inch above the eyebrows in front and at a point in the back of the head that results in the largest possible measurment. Take several measurments. The largest measurment is the one you want to use.