This post applies to men’s and women’s sizing. Because I reported on hats as a trend for men at South By Southwest, I made this a Guy Friday.
Before getting started: Find a cloth or flexible tape measure, not one of those contractor-grade metal ones from the hardware store. You want something intended for use in garment-making because they’re made to measure curves. They cost maybe $2 at the drugstore or fabric store and you should own one anyway for sizing your inseam, waist and other measurements when shopping online.
There is an excellent step-by-step guide to measuring your hat size here. One thing to consider is the amount of hair you have. If you just shaved your head but normally have a lot of hair or if you have a lot of hair and are considering cropping it off, remember that hair takes room in that hat and could change the size by a half-inch or more. If you will be wearing a wig with this hat (you know, like Napoleon or a drag queen or whomever), measure your head wearing the wig.
A tighter hat will stay on in the wind but a looser hat is more comfortable to wear as well as merciful in warmer temperatures.
If you already own a hat that you think fits great, take a tape measure and rest the bottom edge of the tape against the inside of the hat, aligning the base of the tape with the base of the hat. You should also take a measurement of your head to make sure it’s not vastly different from the hat. If they are your hat is probably too big (I cannot imagine anyone enjoying a tight hat). A better fitting hat should have a more proportional shape for your head.