Using a tape measure of plastic or cloth, measure the circumference of your head just above the top of the ears and just above the brow.
Make sure the tape lies midway on your occipital bone (the small bone in the centre back of your head).
Make it a firm measurement, not too tight or loose. Let it sit how you’d like the hat to sit.
If you are measuring up for a leather hat, I will account for the inevitable give in the leather after a few wears of the hat. This hat does need to be a snug fit at first to allow for the give.