Traditionally women's head sizes are measured around the head while men's sizes are measured through the head to front to back. So a woman can measure her head using a soft tape measure, around the skull around 1/2" above the eyebrow, right where a hat sits, while a man should find a caliper to measure his from front to back.
In the interest of equality, let's make that simple and have both sexes measure their heads around. For those of you who know your traditional men's size, here's a conversion chart:
