Hat Size Calculator
Head circumference to US, UK and European hat sizes.
Convert a head circumference into US, UK and European hat sizes and the S/M/L equivalent, with the fractional inch sizing explained.
A centimetre above the ears and across the middle of the forehead, where a hat sits.
US hat size
7.25
L · EU 58
US hat sizes are the head's diameter in inches, which is why they look like odd fractions — a 58 cm head divided by pi is about 7¼. UK sizes run one eighth smaller than US for the same head.
How the Hat Size Calculator works
US hat sizes look like odd fractions because they are: the size is the diameter of the head in inches, derived by dividing the circumference by pi. That is why a 7¼ hat has nothing obviously to do with the 58 cm head it fits, and why the European scale, which is simply the circumference in centimetres, is the easier one to shop in.
Also known as: head size calculator · cap size chart
Why US hat sizes are fractions
A US hat size is the diameter of the head in inches, not its circumference. The trade measured the circumference and then divided by pi to get a figure that corresponded to the block the hat was formed on, and the convention stuck. That is why the sizes come out as awkward eighths: 7, 7⅛, 7¼ and so on.
The European scale skipped the division and simply uses the circumference in centimetres, which is why a European hat size is a recognisable number like 58 while the US equivalent is 7¼.
UK sizes use the same diameter-in-inches idea as the US but run one eighth smaller for the same head, a difference small enough to be easy to miss and large enough to matter on a fitted hat.
A worked example
A 58 cm head is 22.83 inches around. Dividing by pi gives 7.27 inches of diameter, which rounds to the nearest eighth as 7¼ — the US size. The UK size is one eighth below, 7⅛. The European size is simply 58, and the letter size is a large.
Note how coarse the letter sizing is by comparison: S, M, L and XL cover roughly two centimetres of circumference each, so a letter-sized hat is a fit compromise where a numbered one is not.
Where the answer misleads
Head shape varies as much as head size. Two people with identical 58 cm circumferences can have quite different oval shapes front-to-back, and a hat blocked for one will pinch the other at the temples while gapping at the back. Circumference predicts size, not shape.
Structured hats and knitted ones behave differently. A felt or straw hat holds its blocked size and must fit; a beanie stretches and a single size covers a wide range. Sizing charts for the two are not comparable.
Hats also stretch with wear and shrink if they get wet and dry unevenly, so a hat that fits perfectly on day one may not in a year. Where you are between sizes, the larger with internal sizing tape is easier to correct than a small one you are trying to stretch.
Frequently asked questions
How do I measure my head for a hat?
Run a tape around the head about a centimetre above the ears and across the middle of the forehead, where a hat actually sits. Keep it level and snug but not tight. That circumference is the European size directly.
Why are US hat sizes fractions like 7 1/4?
Because the size is the head's diameter in inches rather than its circumference. Dividing a 58 cm circumference by pi gives about 7.26 inches, which rounds to the 7¼ on the label.
What is the difference between UK and US hat sizes?
UK sizes run one eighth smaller than US for the same head, so a US 7¼ is a UK 7⅛. The scales are otherwise identical, which makes the eighth easy to miss.
Should a hat be snug or loose?
Snug enough to stay on in wind, loose enough to leave no mark on the forehead after twenty minutes. If you are between sizes, take the larger and use sizing tape inside, which is easier than stretching a hat that is too small.