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 point for the entire sizing procedure. Due to varying shapes, heads that are apparently the same size when measured by a tape may not necessarily fit the same size helmet. A small metal tape measure or a cloth tape may be used to make your initial measurement. You can also use a string, which can then be laid against a measuring tape. Measure the circumference of the head at a point approximately one inch above the eyebrows in front, and at a point in the back of the head that results in the largest possible measurement. Take several measurements. The largest measurement is the one you want to use.