Measuring your head is 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 cloth tape measure may be used to make your initial measurements. The circumference of your head should be measured at a point approx. 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 to make sure you have the largest one.