Place the measuring tape around your head, about a half-inch above your ears, and across your mid-forehead. This is where most hats will sit naturally on your head.

Make sure the tape is level and snug against your head but not too tight. You should be able to still adjust the tape position without too much friction.

Take note of the measurement in inches or centimeters. It's generally best to round up to the next 1/8 inch or 0.3 cm if you're in between sizes. This will ensure your hat has a little room to breathe and won't leave a red mark on your forehead after a few hours of wear.