GSM stands for grams per square metre. It is exactly what it sounds like: take a square metre of the fabric, weigh it, and write down the number. A 240 GSM jersey means a square metre of that cloth weighs 240 grams.
That is the whole definition. It is worth stating plainly because the figure gets used as a stand-in for quality, and it is not one. It is a measure of mass, and mass is only one of the things that decides whether a shirt is any good.
What a higher number actually buys you
Weight correlates with opacity, with how the fabric hangs, and with how long it takes to lose its shape. A light jersey drapes close to the body and shows what is underneath. A heavier one stands away from you, holds a shoulder line, and takes longer to go slack at the collar and hem.
That is why the number is worth publishing. Two shirts at the same price can differ by eighty grams a metre, and you cannot see the difference in a photograph. You find out in month four.
Where the number stops helping
There is a ceiling. A mill can chase a heavier figure by tightening the knit rather than using better yarn, and past roughly 200 GSM that trade starts to show: the fabric gets denser, stiffer, and warmer without getting better. You end up with a shirt that is technically heavier and practically worse.
So the number on its own is not the answer. It has to be read alongside the yarn, the knit structure and what the fabric is being asked to do. A well-made 190 GSM tee will outlast a badly made 260 GSM one, and no spec sheet will tell you that on its own.
What to ask instead
Ask for the weight, but ask for the rest too. What is the yarn — carded or combed, ring-spun or open-end? Is the collar ribbed and self-fabric bound, or is it a strip of jersey that will wave within a season? Is the number measured, or estimated from the mill’s spec sheet and never checked?
A brand that will answer those questions is telling you something. So is one that will not.
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