Why your size changes between stores

Because no standard binds a brand to a measurement for a given letter, and because the cut of a garment changes it far more than the letter does. A ladies' Medium and a unisex Medium are two different garments wearing the same label. Nothing is broken. The letter was never carrying the information you needed.

What does the letter on the label actually name?

It names a position in one brand's own grade for one style. That is all. There is no rule that makes a Medium measure a set number of inches across the chest, and no body that inspects it. ISO 8559 describes how to take body measurements for garment sizing, and the ASTM body-measurement standards used in the United States are voluntary. Both describe bodies. Neither tells a manufacturer what its Medium must measure once it has been cut and sewn.

So the same letter can name a 17.5" garment in one catalogue and a 20.5" garment in another, and both spec sheets are correct. When you cross from one shop to another you are not moving along a shared scale. You are switching scales entirely, and the label gives you no warning that it happened.

How far apart are two Mediums, really?

Here are six published specifications side by side. Four of them are unisex tees from four different manufacturers. One is a fitted unisex cut. The last is a ladies' cut from the same brand as the first row.

Read the Medium column first. Four unisex styles from four separate manufacturers publish exactly 20.0". The brand is not the variable there. The cut is.

Now read the last row. A Gildan G500L Medium publishes 17.5", against 20.0" for the Gildan G500 Medium sitting in the same catalogue and 20.5" for the Next Level 3600 Medium. That is a gap of 2.5" to 3.0" on the same letter, from the same brand in the first comparison. At Large the gap is a clean 3.0": 19.0" against 22.0". Doubled, that is six inches of circumference between two shirts a shop will both call Large.

The spread across that Medium column is 17.5" to 20.5", or 3.0" between the narrowest and the widest, before any tolerance is applied. That single fact explains most of what people describe as their size changing between stores.

Why does the cut move the number so much?

A ladies' cut is drafted for a different body shape, so it is narrower through the chest, shorter in the body and tighter under the arm. It is a different pattern, not a smaller version of the same pattern. When a shop stocks unisex blanks and another shop stocks ladies' blanks, the two shops are selling different garments under one alphabet.

The trap is that shops rarely put the cut where you can see it. A product page may say Medium and nothing else, and the cut lives three clicks away in a spec sheet, or nowhere. If you learn one habit from this page, make it this one: find the cut before you trust the letter.

What is grading, and why do brands grade differently?

Grading is the step between consecutive sizes. It is chosen by the brand, and brands do not agree on it.

The Gildan G500 grades evenly at 2.0" of flat chest per size from S to XL. The Gildan G500L does not: 1.0" from S to M, then 1.5" from M to L, then 2.0" from L to XL. The Next Level 3600 steps 1.5" from S to M and again from M to L, then 2.0" to XL.

This is why "just size up" is unreliable advice. Sizing up on a G500 buys you 2.0". Sizing up from S to M on a G500L buys you 1.0". The same instruction produces twice the change in one shop and half of it in the other, and neither shop told you which one you were in.

How much of this is ease?

Ease is the room a pattern leaves between the body and the garment. A relaxed style adds more and a fitted style adds less.

Ease also refuses to behave the way the marketing words suggest. The Next Level 3600 is described as a fitted cut, yet at Small it publishes 19.0" flat against 18.0" for the Gildan G500 Small. Fitted describes the shape through the waist and the armhole, not a guarantee of a narrower chest. At Large and XL those two styles publish identical chest figures and still hang differently on the body.

Is vanity sizing to blame for any of it?

Partly, and mostly in the shops where the label carries a body number rather than a garment measurement. Vanity sizing moves the label towards a flattering number while the tape stays where it was, which makes a size on a receipt a poor record of what you actually wore. Our page on vanity sizing sets out how the drift works and why a garment measurement is immune to it.

For the blank tees above the effect is smaller, because those brands publish flat garment measurements rather than an implied body size. That is the whole reason ShirtMath works from garment measurements: a published inch is checkable, and a letter is not.

How much of the gap is manufacturing tolerance?

Less than people hope. Tolerance on published blank specifications runs roughly plus or minus 0.3" to 0.8". Garment-dyed styles carry the wider end of that band, because the dyeing happens after the garment is sewn and the fabric moves in the process. Comfort Colors C4017 is garment-dyed, so plus or minus 0.8" is the sensible allowance there.

Body length carries the same caution. The five unisex and fitted styles in the table above all publish 29.0" of body length at Medium, while the garment-dyed Comfort Colors C4017 publishes 29.5". Half an inch of length is inside the range where tolerance and your own tape can disagree, so treat it as a tendency rather than a fact about the specific shirt in the parcel. The ladies' G500L is a different matter at 26.0", three inches shorter than the unisex row, which is pattern rather than tolerance.

Why do the numbers repeated online drift from the spec sheet?

Because they are copied, not measured. A figure gets lifted from a spec sheet into a forum post, then into a blog, then into a comparison video, and somewhere along that chain a row shifts. One widely repeated comparison of a Comfort Colors Medium against a Gildan Medium quotes a chest figure that belongs to a different size on the published grade, and the conclusion drawn from it is wrong by an inch or two in the wrong direction.

The fix is dull and it works: read the number off the manufacturer's own current specification sheet, note the date you read it, and ignore restatements. That is the rule we hold ourselves to, and our methodology page records where each figure came from.

What should I do before I buy from a new shop?

1. Measure a shirt that already fits — lay it flat, smooth it, and take the chest one inch below the armhole. Our pit to pit guide covers the technique in detail.
2. Find the shop's garment measurements — a flat chest and a body length are enough. If a listing offers only a letter, treat it as an unknown rather than a size.
3. Check you are comparing like with like — flat against flat, circumference against circumference. Doubling one figure and forgetting to double the other is the most common error in this whole exercise.
4. Identify the cut — unisex, fitted or ladies. This decides more than the letter you pick.
5. Allow the tolerance — treat a published figure as a centre point with 0.3" to 0.8" either side, and be wary of choosing between two sizes on a difference smaller than that.
6. Write the numbers down — the chest that works for you is portable between shops. The letter is not.

What else do people ask about this?

Where should I go next?

This page sits in our answers section, alongside the other sizing questions people ask in full sentences.