Most adult head sizes range between 6 ¾ and an 8.

To get your hat size, measure the circumference of your head in inches and divide by pi.

That’s a span of roughly 5 inches, a big difference. So how do people determine a hat size, and why does it seem to fall in such a narrow range of numbers?

The official way to get your hat size in the United States is to measure the circumfrence of your head to the nearest eighth of an inch. Then you divide by pi (3.14). This will give you your official hat size. Every increase in hat size is 4/10ths of an inch.

Store owner Martin Mann said that the smallest adult size hat Hats Plus carries is 6 5/8.

Referring to the largest hat size he’s seen on a customer, manager Tod Canon said, “8 ¼ is the biggest we’ve seen here.”

Mann explained that when dress hats were more commonly worn, hatters made hats in sizes that better fit head shapes called “round,” “oval” and “long oval.” As not every head is shaped the same way, this provided better fits within the same measured size.

Just to make the matter more confusing, there is no universal hat sizing. In the United Kingdom, their sizes are one size smaller than ours. So if you walk into Christie’s to buy a bowler and tell the clerk you’re looking for a 7 3/8, it won’t fit. He’ll sell you a 7 ¼ that fits perfectly.

The French have their own system that designates sizes from zero to 9 ½ . The rest of Europe tends to stick with simply measuring the circumference of your head in centimeters. That is why your City Sport cap (from Belgium) and your Borsalino fedora (from Italy) list their hat sizes in centimeters.