Hat Size Calculator
Convert your head circumference into a US fitted hat size, UK size, EU/centimetre size and an S–XL band.
How it works
US fitted size = head circumference (in) ÷ π · UK = US − 1/8 · EU/cm size = head circumference in cm
A fitted hat is sized by the diameter of the head opening, so the US and UK numbers are just the head circumference in inches divided by π (about 3.14) — the same relationship between a circle’s circumference and its diameter. The UK scale is offset by one-eighth of an inch. Europe skips the conversion entirely and labels hats by head circumference in centimetres, which is why an EU hat size looks like a much bigger number.
Worked example
A 56 cm head is 56 ÷ 2.54 = 22.05 in around. US size = 22.05 ÷ π = 7.02 → 7; UK ≈ 6⅞; EU/cm = 56; and 22 in falls in the Medium band.
Frequently asked questions
How do I measure my head correctly?
Wrap a soft tape (or a string you then measure) around the widest part of your head — about a centimetre above the ears and eyebrows, where a hat naturally rests. Keep it level and snug but not tight.
What if I am between two sizes?
Round up rather than down. A slightly loose fitted hat can be padded with sizing tape inside the band, but one that is too small will not sit properly and leaves a mark.
Why is my European hat size such a big number?
European hats are labelled by head circumference in centimetres, so a size like 56 or 58 is just how many centimetres around your head is — not a different, larger head.
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This calculator is for educational purposes. Double-check important results before acting on them.