What your shirt size means: four sizing languages, decoded

By industry convention — and only convention — a men's M fits roughly a 38–40-inch chest, and a women's M is a US 8–10. An XL in women's numbers is a 16–18. A dress shirt size like 16 34/35 is a 16-inch neck with a sleeve cut for a 34-to-35-inch arm. A European 42 is a 16.5-inch collar, which lands in Large territory. Four sizing languages hang on the same rack. This page decodes each one into plain inches, then shows — in manufacturers' own published numbers — why the letter is the least trustworthy of the four.

Each system is a naming scheme laid over garment dimensions. The dimensions are facts anyone can check with a tape; the names are conventions a brand may follow, bend, or quietly ignore. ShirtMath's own data is garment measurements taken flat, in inches, to one published standard, so where this page shows catalog numbers they are the makers' published specifications, and where it shows convention tables it says so plainly. It sits alongside the rest of the measuring and fit guides.

Alpha sizing names a bracket, not a shirt. For men's and unisex garments, each letter is conventionally marketed against a range of body chest circumference — the "fits chest 38–40" line printed on blank-apparel size charts. That mapping is an industry custom, not a standard anyone enforces, and no law defines it. The table pairs the customary brackets with something checkable: the published flat chest width of one real style, the Gildan G500 tee, from ShirtMath's catalog.

Doubling the flat width gives the garment's circumference: 36, 40, 44, and so on up to 64 inches — each one sitting at the very top of its marketed bracket. That is the mechanism hiding inside the letter. A 38-inch chest in a G500 M has about two inches of room; a 40-inch chest in the same shirt has roughly none. The bracket tells you which shirts to consider, and nothing about how any of them will actually feel.

Women's letters translate differently: not to chest ranges but to the US numeric run of 0–24. The common answer to what M is in numbers is 8–10; for ladies' cuts, XL in numbers is 16–18. This mapping is likewise industry convention, and brands routinely shift it by one step in either direction.

The bust, waist and hip measurements conventionally behind each numeric size are laid out in the women's clothing size chart, along with where the mapping commonly breaks.

A dress shirt size is two measurements wearing one label. The first number is the neck: the shirt's collar, in inches, sized to close comfortably around a neck of that circumference. The second is the sleeve: the distance from the center back of the neck, over the shoulder, down to the wrist, in inches. So 16 34/35 is a collar for a 16-inch neck with a sleeve for a 34-to-35-inch arm — the slash means one sleeve length cut to serve both, a manufacturing economy, not extra precision.

This is the most honest sizing language on the rack, because both numbers are actual dimensions. It still covers only two of a shirt's measurements — chest, length and shoulder hide behind the cut name (slim, regular, classic) and vary freely. The neck-to-letter mapping below is industry-standard reference; ShirtMath holds no dress-shirt measurements of its own.

The full grid — neck, sleeve, and the fit-name problem — is in the men's dress shirt size chart.

The European system is the same neck measurement in different units: the collar number is the neck circumference in centimeters. Multiply inches by 2.54 and round — a 16.5-inch collar is 41.9 cm, sold as a 42. That is the entire code. It reads as cryptic only because the unit is unstated on the tag.

This table is industry-standard reference, and the alpha column is the loosest part of it — brands draw the letter boundaries differently. Conversions for the other numeric systems that show up on imported garments are collected in the international size conversion chart.

Every table above translates between naming systems. This one measures the names against reality. Below is the published flat chest width — the makers' own specification sheets — for every style in ShirtMath's catalog that carries manufacturer-published specs, at one size: M.

The same letter names a 17.5-inch shirt and a 20.5-inch shirt: a 3-inch spread, or six inches of circumference. The gap is mostly the ladies' cut — Gildan's G500L M is 2.5 to 3 inches narrower than every unisex M on file, and nothing on either tag says which dialect it speaks. At L the spread widens to 3.5 inches: 19.0" for the ladies' cut against 22.0–22.5" for the unisex styles. Even among unisex shirts that agree at M, published chests at S run from 18.0" to 19.0", and lengths diverge by up to an inch at XL.

So the letter is a name, not a number. It compresses eight measurements into one character, and the compression is lossy in exactly the dimension you care about. How the labels drifted this far — and why nothing pulls them back — is the subject of the vanity sizing guide; the full eight-style comparison at every size is in the t-shirt measurements chart and which blank runs biggest.

The systems stop being confusing the moment you hold your own numbers, because every one of them converts to inches and inches convert to each other.