Hat sizes are the last place America still measures heads by diameter in inches, which is why the numbers look like nonsense. Nobody's head is 7 1/4 of anything you can see. But once you know the trick (it is your head's diameter, and we get there by dividing by pi), the whole system snaps into focus, and converting between US, UK, and EU sizing becomes one measurement and a little arithmetic. This page does the arithmetic.
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How to measure your head
Take a soft sewing-style measuring tape and wrap it around your head about 1/8 inch (1 cm) above your ears and across the middle of your forehead. That is where the sweatband of a hat actually sits, so that is the number that matters. Keep the tape level all the way around and snug but not tight; do not pull it into your hair or squash it down. No soft tape? Wrap a piece of string around the same line, mark where it meets, and lay the string against a ruler. Measure twice. If the two readings disagree, believe the larger one.
How the calculator works
A US numeric hat size is literally your head's diameter in inches. Old American hatters sized their wooden hat blocks by diameter, so they took the circumference and divided by pi, then rounded to the nearest eighth of an inch. That convention never went away. The UK uses the same idea but labels everything one eighth smaller, and the EU (and most of the world) skipped the geometry lesson entirely and just uses your circumference in whole centimeters.
The formula
UK size = US size − 1/8
EU size = circumference in whole cm
Circumference is the measurement you took above the ears and across the mid forehead. Pi is 3.14159, the ratio of any circle's circumference to its diameter, which is why dividing by it turns your head measurement into the diameter-style US size.
Worked example
Full conversion table, 21 to 25 inches
Every row here is computed with the exact formulas above. Between-size heads should read the next row down (that is, up in size).
| Head (in) | Head (cm) | US size | UK size | EU size | Letter band |
|---|---|---|---|---|---|
| 21 | 53.3 | 6 5/8 | 6 1/2 | 53 | Below S |
| 21.5 | 54.6 | 6 7/8 | 6 3/4 | 55 | S |
| 22 | 55.9 | 7 | 6 7/8 | 56 | M |
| 22.5 | 57.2 | 7 1/8 | 7 | 57 | M |
| 23 | 58.4 | 7 3/8 | 7 1/4 | 58 | L |
| 23.5 | 59.7 | 7 1/2 | 7 3/8 | 60 | XL |
| 24 | 61 | 7 5/8 | 7 1/2 | 61 | XL |
| 24.5 | 62.2 | 7 3/4 | 7 5/8 | 62 | XXL |
| 25 | 63.5 | 8 | 7 7/8 | 64 | XXL |
The letter bands: S is 6 3/4 to 6 7/8 (21.25 to 21.625 in), M is 7 to 7 1/8, L is 7 1/4 to 7 3/8, XL is 7 1/2 to 7 5/8, and XXL is 7 3/4 and up.
Fitted vs adjustable, and what to do between sizes
If you are buying an adjustable cap, relax: the strap or snapback covers most of the adult range and your letter band is all you need. Fitted hats are the reason this page exists. A fitted baseball cap, a felt fedora, or a flat cap needs the real number, and being off by an eighth is the difference between comfortable and a red line across your forehead.
Between sizes? Round up. Hats shrink and heads do not. Wool and felt tighten with humidity, sweat, and time, and a slightly roomy fitted hat is easy to fix with foam sizing tape tucked inside the sweatband. A hat that starts snug only gets worse. If a brand is known to run small (many fashion fitteds do), that is a second reason to take the larger size.
Sizing other things today? Try the ring size calculator or the shoe size converter.