Hat Size Calculator

Background.

A hat size calculator converts one measurement — the circumference of your head at its widest path — into whichever numbering a shop uses. There are three in circulation and they are all describing the same tape reading. The US and UK number a fitted hat in whole sizes and eighths, 6 7/8, 7, 7 1/8, and that number is the nominal diameter of your head in inches: your circumference divided by pi. The continental European number is simply your head girth in centimetres, which is how EN 13402-3 says a body-measurement label should be expressed. Letter sizes band the same centimetre figure into S, M, L and XL.

There is a genuinely elegant fact hiding in the relationship between the first two, and it explains why hat sizing feels tidier than the rest of apparel. One eighth of a US hat size is pi divided by eight inches of circumference, which works out to 0.99748 centimetres — within a quarter of a percent of exactly one centimetre. So one centimetre of head is one eighth of a hat size, and the two scales are the same scale in different clothing: a 56-centimetre head is a US 7, 57 is 7 1/8, 58 is 7 1/4, 60 is 7 1/2, 64 is a US 8. Divide the continental number by eight and you have the US size, exactly, all the way across the range. This page checks that identity across every centimetre from 40 to 70 as a test, which is a stronger guarantee than any chart can offer.

One thing this page does that others do not is tell you what is not behind the convention. A number of hat size pages assert that ASTM D6661 defines the US hat size as head circumference divided by pi. ASTM D6661 is "Standard Practice for Field Collection of Organic Compounds from Surfaces Using Wipe Sampling" — verified against ASTM's own catalogue. It has nothing whatever to do with headwear, and the claim appears to have been copied between sites until it acquired the look of a citation. No ASTM, ISO or CEN standard defining the US hat scale could be found at all. What the circumference-over-pi rule has going for it is not a standard but reproducibility: it returns the published anchor at every row of every hat chart, and the centimetre identity above explains why. That is a good reason to trust it and a bad reason to call it a specification, and this page says so beside the answer rather than in a footnote.

Measuring well matters more here than on most sizing pages, because a fitted hat has no adjustment. Take the tape round about a centimetre above the ears and across the middle of the forehead — the widest path, not the narrowest — and keep it snug and level. Hair volume is a real variable: a haircut can move a fitted size by an eighth, which is exactly one centimetre of circumference. If your unrounded size lands close to halfway between two eighths, this page says so and tells you to try both, because that is a case where the arithmetic genuinely cannot decide for you.

The page labels head girths from 40 to 70 centimetres and warns you when you fall outside 53 to 64, the span adult fitted hats are generally stocked in. Below it you are in youth and child ranges, which are graded separately; above it you are into specialist and made-to-order blocks. It does not model head shape at all, and that is a real limitation for fitted hats: long-oval and round-oval heads of identical circumference fit differently in the same crown, which is why traditional hatters keep a conformateur and a stretcher rather than only a tape.

What is hat size calculator?

A hat size is a description of head circumference in one of three notations. The US and UK fitted scale expresses it as the nominal head diameter in inches — the circumference divided by pi — in steps of an eighth of an inch of diameter, which is why the numbers run 6 3/4, 6 7/8, 7, 7 1/8. The UK scale is the same idea offset by one eighth, so a US 7 1/8 is a UK 7. The continental scale is the head girth in centimetres itself, in steps of one centimetre, matching the body-dimension labelling principle EN 13402-3 sets out for European size labels. ISO 8559-1:2017 is the authority for how head girth is located and measured.

The three notations agree because their steps agree: an eighth of an inch of diameter is pi over eight inches of circumference, or 0.99748 cm, so one continental step and one US step are the same amount of head to within a quarter of a percent. The consequence is a clean division: the continental number divided by eight is the US size.

What no standard supplies is the US scale itself. Unlike glove sizing, where EN ISO 21420 publishes an explicit table, the US and UK hat numbering is a trade convention with no standards document behind it that could be located — and at least one standard number circulating as its authority is a misattribution. It is nonetheless internally consistent and reproduces every published chart, which is the honest basis on which to use it.

How to use this calculator.

1. 01Pick your unit — inches or centimetres.
2. 02Measure your head circumference: tape round the head about a centimetre above the ears and across the middle of the forehead. That is the widest path, and it is the one hat sizes are graded from.
3. 03Keep the tape snug and level all the way round, and measure over your hair as you normally wear it.
4. 04Choose the labelling system for the shop you are buying from — continental centimetres, US eighths, UK eighths, or letters.
5. 05Read the size. Check the unrounded figure too: if it sits close to halfway between two eighths the page will tell you, and you should try both.
6. 06Remember that one centimetre of head is one eighth of a hat size. That is how much a haircut can move you.
7. 07For a fitted cap with no adjustment, buy from a retailer that will exchange it, or prefer an adjustable style if you are on a boundary. Head shape — long oval versus round oval — affects fit at identical circumference and no calculator can see it.

The formula.

The circumference is converted to both inches and centimetres up front, because each scale is defined in its own unit. The continental number is the centimetre figure rounded to the nearest whole centimetre. The US number is the inch figure divided by pi — giving the nominal head diameter, which is what the fitted scale numbers — then rounded to the nearest eighth, half-up. The UK number is exactly one eighth below the US number.

The unrounded value is published as its own output, because on this page the rounding step is the whole game. One eighth of a hat size is only 0.99748 centimetres of circumference, so an eighth of an inch of tape slack moves you a size. When the unrounded figure sits within a tenth of an eighth of the midpoint between two sizes, the page says so and tells you to try both — the arithmetic genuinely cannot decide, and pretending otherwise would be worse than admitting it.

That same 0.99748 figure is the reason the two numeric scales agree so cleanly. Because one continental step of one centimetre and one US step of one eighth are the same amount of head to within a quarter of a percent, the count of eighths in your US size and your continental centimetre number are the same integer: US size equals the continental number divided by eight. A 56-centimetre head is a US 7 because 56 ÷ 8 = 7; a 58 is 7 1/4 because 58 ÷ 8 = 7.25. The test suite asserts this for every whole centimetre from 40 to 70, so the two scales cannot drift apart under a future edit.

The letter bands read the unrounded centimetre value, not the rounded continental number, and this matters at the edges: a 56.9-centimetre head rounds to continental 57 but bands as S, because M begins at 57.0 centimetres. Classifying the rounded number would have printed "57 / M" for a head that is genuinely below the M threshold. The bands are: under 55 centimetres XS, 55 to under 57 S, 57 to under 59 M, 59 to under 61 L, 61 to under 63 XL, 63 and above XXL, and every edge is tested below and at.

Worked through with the page's own example: a 58-centimetre head is 58 ÷ 2.54 = 22.8346 inches. Divided by pi that is 7.26849, which multiplied by eight is 58.148 eighths — rounding to 58 eighths, or 7.25, printed as 7 1/4. The UK size is 7 1/8, the continental number is 58, and the letter band is M. Note that 58 eighths and 58 centimetres are the same integer, which is the identity above showing up in the worked example itself.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Organization for Standardization, ISO 8559-1:2017 — Size designation of clothes, Part 1: Anthropometric definitions for body measurement. The authority for how head girth is located and measured. Sold by ISO; catalogue entry consulted.
2. [2]European Committee for Standardization, EN 13402-3:2017 — Size designation of clothes, Part 3: Size labelling based on body measurements and intervals. The principle that a European size label expresses the body dimension itself in centimetres, which is what a continental hat number is. Sold by CEN national members; abstract and scope consulted.
3. [3]ASTM International, ASTM D6661-17 — Standard Practice for Field Collection of Organic Compounds from Surfaces Using Wipe Sampling. Cited here to correct a false claim: several hat-size pages assert that D6661 defines the US hat scale. This is the actual document at that designation, and it concerns wipe sampling for organic compounds, not headwear.
4. [4]National Institute of Standards and Technology, "Withdrawn DOC Product Standards (PS)" — the register of withdrawn US Department of Commerce apparel sizing standards. None of the four apparel standards listed covers headwear, and all are withdrawn; there is no US government hat sizing standard.