To determine your correct size, you first need to understand that hats come in three general sizes:
The most commonly used sizing are SM, MD, LG, XL and occasionally 2 & 3X sizes.
The more traditional sizing is that of the exact size in inches and/or in centimeters. You will commonly see these sizes on more expensive hat. It is important to note that hat size correspondents are not always exactly matching between brands and styles and every hat is a little bit different. Always have a hat liner readily available should you hat be a little larger than you'd like!