Hat Sizes
Authority: Traditional US/UK Hat Sizing Convention
Hat sizes are based on head circumference measured about 1cm above the ears. US sizes represent head diameter in inches, calculated by dividing circumference in centimeters by 8 -- so a 56cm head is a US size 7. UK sizes use a related but not identical numeric scale.
| US Size | UK Size | Head Circumference (cm) | Head Circumference (in) |
|---|---|---|---|
| 6 3/8 - 6 1/2 | 6 1/2 - 6 5/8 | 51 - 52 | 20 1/8 - 20 1/2 |
| 6 5/8 - 6 3/4 | 6 3/4 - 6 7/8 | 53 - 54 | 20 5/8 - 21 1/4 |
| 6 7/8 - 7 | 7 - 7 1/8 | 55 - 56 | 21 5/8 - 22 |
| 7 1/8 - 7 1/4 | 7 1/4 - 7 3/8 | 57 - 58 | 22 1/2 - 22 7/8 |
| 7 3/8 - 7 1/2 | 7 1/2 - 7 5/8 | 59 - 60 | 23 1/4 - 23 5/8 |
| 7 5/8 - 7 3/4 | 7 3/4 - 7 7/8 | 61 - 62 | 24 - 24 3/8 |
| 7 7/8 - 8 | 8 - 8 1/8 | 63 - 64 | 24 3/4 - 25 1/4 |
How Hat Sizes Are Calculated
Hat size starts with a single measurement: head circumference, taken with a tape measure wrapped around the head about a centimeter above the ears, at the point of maximum circumference. US hat sizes translate that measurement into head diameter, expressed in inches. Because a circle's diameter equals its circumference divided by pi, and one inch equals 2.54cm, multiplying pi by 2.54 gives almost exactly 8 -- so US hat size is commonly calculated simply by dividing head circumference in centimeters by 8. A 56cm head, for example, works out to size 7 (56 / 8 = 7).
UK hat sizing measures the same underlying quantity but was developed independently, using a slightly different numeric scale that doesn't always land on the same fraction as the US system for a given head. That is why hat retailers publish conversion tables rather than a single universal size number: unlike shoe sizing, which now has an international Mondopoint standard, hat sizing was never unified under a single modern global standard -- the ISO standard that once covered headwear sizing, ISO 4417, has since been withdrawn.