To properly determine your cap size, measure the circumference of your head at the widest part, or where your caps generally sit, using a soft measuring tape or string and ruler. See the chart below to determine your size. (Measurements are all +/- 1/8")
I recommend buying a cap that will fit snug (not tight) upon the first wear. With just a little bit of time, the cap will shape and mold to your head, providing the perfect, comfortable fit.
*Everyone wears his or her cap differently and every manufacturer has different size specifications. I developed my size chart based on inch measurement with 3/8" increments between sizes.