US fitted
circumference (in) ÷ π, to the nearest ⅛
One measurement, four labels
Hat Size Chart & Converter — US, UK, EU, S/M/L
A hat size is not an arbitrary code. The US number is your head's diameter — circumference in inches divided by π — which is why it comes out as 7 1/4 rather than a round number. The UK runs one eighth lower, and Europe just prints the circumference in centimetres. Those three are arithmetic on a single measurement, so they never disagree. The letter on the label is a different animal: S/M/L is a two-centimetre bucket with no standard behind it, which is how the same head is a medium at one brand and a large at the next. This converter works from the measurement and tells you when you are sitting on a bucket's edge.
The three definitions
UK
US − ⅛
Europe
circumference in cm
What this is good for
Buying a fitted cap online
Fitted caps are sold in US eighths with no give. Measure once and you can order any of them without guessing which letter the brand means.
Ordering from a European maker
European hats are labelled in centimetres, which is the measurement itself. No conversion is needed once you know your number.
Deciding whether to trust S/M/L
If your circumference sits within a centimetre of a bucket edge, the letter is a coin flip between brands. That is exactly when you should buy on the number instead.
Frequently asked questions
Why is my hat size a fraction like 7 1/4?
Because it is a diameter in inches, not a code. Your head circumference in inches divided by π gives roughly 7-ish for most adults, and the trade rounds it to the nearest eighth.
Is a European hat size the same as head circumference?
Yes. A European 58 is a 58 cm head. That is the whole conversion.
Why do UK sizes run smaller than US?
The two trades adopted the same diameter idea but different reference points, leaving a fixed one-eighth offset. It applies across the whole range, so it is easy to correct for.
Can I trust the S/M/L on the label?
Less than the number. There is no standard defining where medium ends and large begins, so brands set their own boundaries. Use the letter as a hint and the circumference as the decision.
Sources and method
US fitted sizing is head circumference in inches divided by π, rounded to the nearest eighth; UK sits one eighth below; European sizing is the circumference in centimetres. Checked against a published fifteen-row conversion chart running from 50 cm to 64 cm — every row matched, including the UK offset. The alpha bands shown here follow the same chart's two-centimetre steps, which is a common convention rather than a standard.
Circumference is only half of fit. Crown shape — long oval against round — changes how a hat sits at identical measurements, so treat this as a starting size rather than a guarantee.