US fitted sizes are a diameter
A US hat size such as 7 ⅜ is the diameter of your head in inches, as if your head were a perfect circle. Hatters measure the circumference and divide by pi (3.1416). A 23-inch head gives 23 ÷ 3.1416 = 7.32, which rounds to 7 ⅜.
Sizes go up in eighths because that was the smallest step a hatter could block reliably. One eighth of diameter is about ⅜ inch, or 1 cm, of circumference.
UK sizes are one eighth smaller
British hatters measured the inside of the hat block, which is slightly smaller than the head. The result is a system that runs one eighth below US sizes, so a US 7 ⅜ is a UK 7 ¼.
European and Asian sizes are centimetres
In Europe, Japan and Korea the size is simply the circumference in centimetres. A 59 cm head wears a size 59. It is the easiest system to use because no conversion is needed.
Letter sizes are ranges
S, M, L and XL each cover about two fitted sizes, and every brand draws the lines in a slightly different place. That is why this site lists sizes by brand as well as by system.
| cm | Inches | US | UK | Letter |
|---|---|---|---|---|
| 46 | 18 ⅛ | 5 ¾ | 5 ⅝ | – |
| 47 | 18 ½ | 5 ⅞ | 5 ¾ | – |
| 48 | 18 ⅞ | 6 | 5 ⅞ | – |
| 49 | 19 ¼ | 6 ⅛ | 6 | – |
| 50 | 19 ⅝ | 6 ¼ | 6 ⅛ | – |
| 51 | 20 ⅛ | 6 ⅜ | 6 ¼ | – |
| 52 | 20 ½ | 6 ½ | 6 ⅜ | XS |
| 53 | 20 ⅞ | 6 ⅝ | 6 ½ | XS |
| 54 | 21 ¼ | 6 ¾ | 6 ⅝ | S |
| 55 | 21 ⅝ | 6 ⅞ | 6 ¾ | S |
| 56 | 22 | 7 | 6 ⅞ | M |
| 57 | 22 ½ | 7 ⅛ | 7 | M |
| 58 | 22 ⅞ | 7 ¼ | 7 ⅛ | L |
| 59 | 23 ¼ | 7 ⅜ | 7 ¼ | L |
| 60 | 23 ⅝ | 7 ½ | 7 ⅜ | XL |
| 61 | 24 | 7 ⅝ | 7 ½ | XL |
| 62 | 24 ⅜ | 7 ¾ | 7 ⅝ | XXL |
| 63 | 24 ¾ | 7 ⅞ | 7 ¾ | XXL |
| 64 | 25 ¼ | 8 | 7 ⅞ | 3XL |
| 65 | 25 ⅝ | 8 ⅛ | 8 | 3XL |
| 66 | 26 | 8 ¼ | 8 ⅛ | 3XL |