How Do I Figure Out the Correct Hat Size to Buy?
It's really not as difficult as it may seem. And, if you already own some caps (fitted or adjustable) it's even easier! If you already own a fitted cap, the size will be on a tag (if not immediately visible, it's most likely inside the sweatband of the cap). Use that size when ordering a new cap (assuming you like the fit, otherwise adjust accordingly). And if you have an adjustable cap (typically with a tag proclaiming "One Size Fits All" (OSFA) or "One Size Fits Most" (OSFM)) then you're good to go with a OSFA or OSFM. Generally speaking, those caps are adjustable from size 6 1/2 up to 7 1/2 (the majority of adult heads fall within this range).
If you're ordering a fitted cap for the first time, you'll need to obtain a cloth measuring tape and measure the circumference of your head (The distance around your head, making sure to measure the same area your sweatband will rest when you wear your cap). Find your number on the chart below. If your number is, for example, 22 3/8" (56.8 cm), then the chart indicates you require a size 7 1/8 fitted cap. You would be considered as an "Adult Medium" size and -- based on the criteria in the first paragraph, could also wear a OSFA or OSFM adjustable cap.
There. That wasn't so bad, was it?
Now, go get that cap!