Please note this is a general guide as different head shapes may also affect your hat size and it can also depend on where the hat sits on your head and the style of the hat too.
If a hat is too small, it will sit too high up on your head so if you are in-between sizes, it's generally best to go up a size and add some hat padding. Some hats have a sweatband that you can place padding under to help it fit a bit tighter.
We sell hat adjuster pads which can be found here if your hat is a little loose and has a sweatband.
International Hat Size Conversion Chart:
Here's a quick video to show you how to measure to find out your hat size: