Wrap a tape measure horizontally around your forehead. Select the helmet that is the closest fit to that size. A helmet should fit snug on your head. If it moves while wearing, it is too large. If it is painful to wear, it is too small.
Comment: By my military hat size I should be a medium according to the chart, but in actuality my narrow head fit best in an extra small. Seams like it may be especially hot in summer. I can never tell if the vents are open or closed because doesnt seam to make any change in air flow.