Hat Size Converter

Head circumference in centimetres and inches, the US fitted scale, the UK scale, the EU number and S/M/L — all from one measurement. The US size is your head’s DIAMETER in inches, which is where the odd numbers come from, and the UK scale sits an eighth of an inch below it.

The fitted size is a diameter

Every size, from one measurement

The fitted scale itself

Why circumference is not enough

Why hat sizes are 7⅛ and not 57

A US fitted hat size is not a circumference. It is the diameter of your head in inches, written in eighths, and it exists because a nineteenth-century hatter measured the wooden block rather than the customer. Divide a head’s circumference in inches by π and you have the size: 22 in round is 7.00 across, so a size 7; 22.44 in round is 7.14 across, so a 7⅛. Once you know that, the whole scale stops being arbitrary — and the arithmetic is checkable, because computing it from nothing but the circumference reproduces two independently published charts at all 21 of their rows.

The eighth-inch step is almost exactly one centimetre of circumference. π ÷ 8 in = 0.9975 cm. That is a pleasing accident and a useful one: the European scale, which is simply the circumference in centimetres, has the same resolution as the American fitted scale, so 56 cm ↔ 7, 57 cm ↔ 7⅛, 58 cm ↔ 7¼ holds all the way up and down without drifting. It also tells you how coarse a fitted size is: one centimetre of head. A tape can distinguish finer than the sizes can.

The British scale is an eighth lower, and nobody can say why. At the same head, a US 7⅛ is a UK 7. Two published charts and a working milliner all state the offset, so it is real and it is what this page uses. What no source consulted could produce was a documented reason for it — the usual suggestion is a different allowance for the internal ribbon, but that is folklore rather than a citation, so this page does not assert it. And one chart, size.ly’s, does not show the offset at all above 55 cm: it prints the British and American numbers as equal. That disagreement is printed in the table above rather than quietly resolved, because a reader ordering a hat from the other country needs to know the risk is one size in the fine-grained direction.

Circumference will not get you a fit on its own, because heads are not round. Hatters distinguish a regular oval head from a long oval one: if your front-to-back length exceeds your side-to-side width by more than a centimetre, you are long oval, and between a fifth and a third of people are. A long oval head in a regular oval block presses at the forehead and the back of the skull while gapping at the temples — at the correct circumference. Stock hats are blocked regular oval. If a hat in your size has always felt wrong in that particular way, that is the reason, and it is worth asking a hatter for the other block. Enter both axes above and the page will tell you which you are, and check them against the circumference you gave.

Measuring, briefly. Tape round the head where the hat will actually sit: about half an inch above the eyebrows, just above the ears, round the widest part at the back. Level, snug, one finger under the tape. Measure with your hair as you will wear it. Then remember that makers stray from their own labels — one hat specialist reports Mayser running about a centimetre large and Italian hats running small — so a half-size of allowance and a felt sizing strip are normal tools rather than admissions of defeat. For the other body scales built the same way, see the clothing size converter and the shoe size converter. This page does the arithmetic. Where the answer carries a consequence — a dose, a structural load, a legal area, a medical measure — confirm it against the authority that governs it before relying on it.

Frequently asked questions

Related calculators

References

1. lazytools.io, Hat Sizes Decoded: Why 7⅛ Means Your Head’s Diameter (read 26 September 2026): the chart this page’s US column is checked against, and the statement that the US fitted size is the head circumference in inches divided by π, with the UK size an eighth of an inch below it.
2. size.ly, Hat Size Chart (read 26 September 2026), thirteen rows from 53 to 65 cm. Its US column agrees with the divide-by-π rule at every row. Its UK column does NOT show the eighth-of-an-inch offset above 55 cm — it makes UK and US equal — which is the disagreement this page reports rather than resolves.
3. sizechartbase.com, Hat Size Chart (read 26 September 2026), which states the offset in words: “British fitted sizing uses the same imperial scale but runs an eighth of an inch smaller, so a US 7 1/4 corresponds to a UK 7 1/8”, and gives the letter-size centimetre bands used here.
4. Eric Javits, Hat Sizing Guide (read 26 September 2026) — a milliner’s own guide, and a third source for the eighth-of-an-inch offset between the British and North American scales. Its letter bands (small 54 to 55 cm, medium 56 to 57, large 58 to 59, extra large 60 to 61) match sizechartbase’s.
5. Agnoulita Hats, Head Shape Calculator: Regular vs Long Oval (read 26 September 2026): a head is long oval when its front-to-back length exceeds its side-to-side width by more than 1 cm (3/8 in), and 20 to 30 per cent of heads are. A long oval head in a regular oval block presses front and back and gaps at the sides — at exactly the right circumference.
6. Brim on Fifth, Navigating the Nightmare World of Hat Sizes (read 26 September 2026), on how far makers stray from their own labels: Mayser hats run about a centimetre bigger than the stated size and Italian hats run smaller.