What this calculator does
US hat sizes are head circumference in inches divided by π, which is to say the diameter of a circle with that circumference. A 57 cm head is 22.4 inches around, giving a hat size of 7 1/8.
UK sizes run one eighth smaller than US sizes for the same head, which catches people out when ordering across the Atlantic. The 57 cm head that takes a US 7 1/8 takes a UK 7. Letter sizes are coarser again, with M covering roughly 56 to 57 cm, so two people in the same letter size can be a full numeric size apart.
The formula
The head circumference is converted to inches and divided by π to give the US hat size, which is effectively the head diameter. The UK size is one eighth smaller, and the letter size comes from the standard centimetre bands. Sizes are quoted in eighths, so most results round to the nearest eighth rather than landing exactly.
| Term | Meaning |
|---|---|
| US hat size | Head circumference in inches divided by π, equal to the head diameter. |
| UK hat size | One eighth smaller than the US size for the same head. |
| Letter size | S, M, L and so on, based on centimetre bands. Coarser than numeric sizing. |
| Head circumference | Measured around the widest part, just above the ears and across the mid forehead. |
The inputs explained
| Field | What to enter |
|---|---|
| Head circumference (cm) | Head circumference in centimetres, taken around the widest part of the head: just above the ears and across the middle of the forehead. Keep the tape snug but not tight. |
When to use it
Buying a hat online
Sizes are quoted numerically, by letter or in centimetres depending on the maker, and the measurement converts to all three.
Ordering from another country
US and UK numeric sizes differ by an eighth for the same head, which is enough to matter in a fitted hat.
Buying a fitted cap
Fitted caps have no adjustment at all, so the numeric size has to be right rather than close.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What hat size does each head measurement take?
A range of head circumferences with the corresponding sizes.
| Head circumference | US hat size | Letter size | UK size (nearest 1/8) |
|---|---|---|---|
| 54 cm | 6.767 | S | 6.750 |
| 56 cm | 7.018 | M | 7.000 |
| 57 cm | 7.143 | M | 7.125 |
| 58 cm | 7.268 | L | 7.250 |
| 60 cm | 7.519 | XL | 7.500 |
| 62 cm | 7.770 | XXL | 7.750 |
Questions
How do I measure my head for a hat?
Run a soft tape around the widest part, just above the ears and across the middle of the forehead, where the hat band will sit. Keep it snug but not tight. Measure twice, since a centimetre of difference is a full eighth of a hat size.
Why is hat size a number like 7 1/8?
Because it is the head diameter in inches: circumference divided by π. A 22.4 inch head has a diameter of about 7.13 inches, hence 7 1/8. The convention dates from when hat blocks were specified by diameter.
What is the difference between US and UK hat sizes?
UK sizes run one eighth smaller for the same head, so a US 7 1/8 is a UK 7. The difference comes from different historical allowances for the hat band. Always check which system a retailer is quoting.
Should I size up or down between sizes?
Up, generally. A hat slightly too large can be adjusted with sizing tape inside the band, which is a cheap and standard fix. One that is too small cannot be made bigger and will be uncomfortable from the first wearing.
For glove sizing, see the glove size calculator. For belts, see the belt size calculator.