There are many size conversion charts on Ebay as to Cowboy Hats.  I have looked at all of them and for the most part, many of them are wrong.  I sell Cowboy Hats on Ebay that come in actual sizes.  The correct way to find out what size you wear, is go 1 inch above the eyebrows and measure around your head.  You can use inches or centimeters.  The Best way is to use centimeters.  For example:  54 CM is a size 6 3/4, 55 CM is size 6 7/8, 56 CM is size 7 and so on.  Hat sizes go every 1/8 on an inch,( 6 3/4, 6 7/8, 7, 7 1/8 and so on.  There is 3/8 on an inch in actual measurement per size.  If your measurement is in between Centimeters, go to the higher CM, example, if your between 57 & 58 CM, get a 58.  If your Cowboy Hat is tight, it will give you a Headache.  You can see these hats listed under Lazycowboy1 or go to Quality-Products-4-You.