Convert hat sizes between US, UK, European (cm) and head circumference.
Apparel • US/UK/EU
Choose the size you already know and the equivalents in all the other standards appear together, with your source highlighted — so nothing has to be converted twice to be checked.
Results are recalculated on every keystroke, with no submit step and no page reload, so a value can be adjusted until the answer looks right rather than guessed once.
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Hat sizes vary by country: the US and UK use fractional numbers, while Europe uses head circumference in centimeters. This chart converts between US, UK, EU and your head measurement.
Measure around your head just above the ears and eyebrows; that circumference in centimeters is your EU size and maps directly to the US and UK sizes shown here.
Hat sizing is unusual in that the US and UK numeric systems encode the head's diameter in inches rather than its circumference, on the assumption that a head is roughly circular. Dividing the measured circumference by π gives that notional diameter, which is the size number. Metric sizing simply quotes the circumference in centimetres, which is both more direct and more accurate, since heads are actually ovals. UK sizes run an eighth smaller than US for the same head, a small but real offset.
US hat size = head circumference (inches) ÷ πMetric size = head circumference in centimetresUK size = US size − ⅛Measure around the head about 1 cm above the earswhere:
Assumptions: Heads are oval rather than circular, so the diameter model is an approximation. Measure with a soft tape held snug but not tight, about a centimetre above the ears and across the middle of the forehead.
Convert a metric measurement into US and UK numeric sizes.
Result57.9 cm = US 7¼ = UK 7⅛ = metric 58
Because one eighth of a size is only about a centimetre of circumference, hat sizing is finer than most clothing sizing and a single size error is noticeable. Fitted caps use the same numeric scale, while "one size fits all" relies on an adjustable strap rather than any measurement.
The traditional US and UK hat numbers look arbitrary until you notice what they measure. They are not a circumference but the diameter of the head, derived by dividing the measured circumference by pi and rounding to the nearest eighth. A 22.5-inch head gives 7.16, which becomes a size 7⅛ — and that is the whole system, which is why hat sizes cluster so tightly between about 6¾ and 7⅝ while circumferences span several inches.
European sizing skipped the arithmetic and simply uses the circumference in centimetres, which is why those numbers run in the fifties and sixties and step in whole units. That makes the metric figure the reliable one to work from: it is the measurement itself rather than a derived and rounded value, so converting from it loses nothing, while converting back into eighths of an inch necessarily rounds.
Fit depends on more than one number, which is where any chart stops helping. Heads are ovals rather than circles, and hatters describe that as the long oval or regular oval block a hat is made on, so two hats of the identical stated size can fit quite differently. Felt and straw also relax with wear while a structured cap does not, which is why the traditional advice is to size to the measurement and adjust with a sizing tape rather than to size up on the chart.