I thought I would post this because I see online auctions all the time where a seller has a hat/cap/beret etc. and doesn't know the size so they lay a tape measure on it and say it "is about 7 inches across". Of course, hats are not rigid and flex and bend so measuring it in this way is pointless. Hat sizes are determined by the circumference around the sweatband (part of the hat that actually comes in contact with the wearer) or opening. In US sizes, this is the circumference in inches divided by Pi (3.14), in Metric sizes it is the opening circumference in centimeters. To convert, multiply the US size by 8 to get the Metric size. For example, my noggin all the way around just above the ears measures about 22 3/4 inches. Divided by 3.14 is 7.25 and that multiplied by 8 is 58. Therefore my hat size is 7 1/4 or 58. This is a handy chart, notice to go from Metric to US just divide by 8 (which is Pi times centimeters per inch, or 3.14 x 2.54=7.9756).