What size am I in a different brand?

The letter stitched into your collar is a label, not a length. Nothing in the alphabet is standardised, so a Medium from one maker can be three inches narrower across the chest than a Medium from another. The way through is arithmetic, not guesswork.

What size am I in a different brand?

You find it by measuring a shirt you already own and comparing that number against the new brand's published measurements. Take the shirt that fits the way you want it to, lay it flat, measure across the chest, then pick the size in the new brand whose published chest figure sits closest to yours. Both sides of that comparison are lengths, so the comparison actually means something.

Converting letter to letter does not work, because there is no shared definition of Medium to convert between. Converting measurement to measurement does work, because an inch is an inch in every factory on earth.

Why does the letter on the label not transfer?

Sizing is set by each brand for its own customers. A blank manufacturer selling to screen printers cuts for a boxy, roomy fit. A fashion label cuts to flatter its house silhouette. A ladies' cut is drafted from a different block entirely. None of them are wrong, and none of them are talking to each other.

Two further forces pull the letters apart. The first is fit intent: a garment described as fitted is drafted narrower than one described as classic, on purpose. The second is the slow drift of labelling over time, which is why the same nominal size has crept larger at many retailers. That drift is covered in the vanity sizing guide, and it is the reason your wardrobe already contains three different Mediums.

Because sizing is set per brand, a size chart that promises to convert brand A to brand B has to invent a relationship that does not exist in the source data. Measurements do not need to be invented. They are published.

Which measurement should I compare first?

Flat chest, often called pit to pit, decides more of how a shirt feels than any other single figure. It is measured seam to seam across the garment laid flat, one inch below the armhole, and almost every blank manufacturer publishes it. Start there, because it is the number most likely to exist on both sides of your comparison.

Body length is the second figure to check, and it is the one that most often makes a shirt feel wrong even when the chest is right. Sleeve length matters for long sleeve styles. Shoulder width and bicep are useful when a brand publishes them, which is less often than you would hope.

If you have not measured a garment before, the how to measure a shirt guide walks through the whole set, and the pit to pit guide covers the chest measurement on its own.

How far apart are the published numbers?

Here are eight blank styles at four sizes, as published by their makers. Read across a row to see one brand's progression, and down a column to see how little agreement there is on a single letter.

Body length at Medium is 29.0" for the unisex styles in this set; the C4017 Medium is the longest at 29.5". A shirt that measures correctly at the chest and short at the hem will still feel like the wrong size.

Why is the ladies cut so much smaller at the same letter?

This is the largest gap in the table and the one that catches people out. The Gildan G500L is a ladies' cut and publishes 17.5" at Medium. The unisex Mediums beside it publish 20.0" to 20.5". That is a three inch spread on a single letter, from one manufacturer, in one catalogue. The gap at Large is wider still, at 3.5".

Nothing has gone wrong there. Medium simply names a different garment depending on the cut, and cut is a separate axis from size. If you are moving between a unisex blank and a ladies' cut, expect to change letter, and let the measurement decide which way.

Is a Comfort Colors Medium two inches wider than a Gildan Medium?

No. That claim is repeated widely and it is wrong. The C4017 Medium publishes 20.5" against the G500 Medium's 20.0": half an inch, not two, and well inside the ±0.8" tolerance a garment-dyed style carries. The 22.0" figure people quote for Comfort Colors is a size Large figure on the unisex ladder, not a Medium, and the C4017's own Large publishes 22.5".

How do I do this in four steps?

1. Pick the reference shirt — the one you would replace if it wore out, not the one you wish fitted.
2. Measure it flat — chest first, then body length, then sleeve if it is a long sleeve style. Smooth the fabric, do not stretch it.
3. Read the target brand's own spec — the manufacturer spec sheet, not a retailer's reprint of it, because reprints go stale.
4. Choose the closest chest figure — then sanity check body length before you decide. If two sizes are equally close on chest, let the length break the tie.

How much error should I expect?

A published figure is a target, not a promise. Manufacturing tolerance on these specs runs roughly plus or minus 0.3" to 0.8" depending on the style. Garment dyed styles such as the C4017 carry the wider end of that band, because the dyeing process shrinks and relaxes fabric unevenly.

Two practical consequences follow. A gap of half an inch between two candidate sizes is inside the noise, so treat it as a tie and break it on length or on how you like the shirt to sit. A gap of two inches or more is real, and you will feel it on the body.

The same caution applies to the tape in your own hands. Measure twice, and take the second reading after smoothing the garment again rather than reusing the first placement.

What if the brand publishes nothing?

Some brands publish body measurements instead of garment measurements, some publish a single chest range per size, and some publish nothing at all. Where only body measurements exist, you cannot compare them directly against a flat garment figure, because one is a circumference around a person and the other is half the width of a piece of cloth lying still.

Where there is no published number, the honest answer is that the size cannot be derived, only estimated from the brand's category and stated fit. Say so rather than trusting a converted letter. ShirtMath leaves a field blank when it cannot read it confidently, for the same reason.

If you want the underlying comparison set, the chest measurement data page lists the published figures, and which blank runs biggest ranks the styles against each other. More questions of this shape live on the answers hub.

Common questions