Take a measuring tape and place it around the circumference of your head, slightly above the ears, (just above where the edge of the hat will sit) with the tape held firmly. It shouldn’t be too tight, just fit snugly against your head.
If you don’t have a tape measure, then use a piece of string or wool to measure around your head and then check the length using a ruler or other measure.
You can give me the size in inches or cm’s… whichever you prefer, I work with both.
If the item is for a gift, it is a great idea to find out if the recipient can wear a baseball cap comfortably. If they find a cap very big or very small, or even just right this will help me to “guess” their size and get that gift right first time!