What does GSM mean on a t-shirt?

GSM stands for grams per square metre. It is the weight of one square metre of the fabric, and it is the closest thing the t-shirt world has to an honest spec. Everything else on a product page is adjectives. GSM is a number you can compare.

Roughly, here is what the numbers feel like in your hands.

The ranges

120 to 150 GSM. Lightweight. Thin, slightly see-through over a dark bra or a tattoo, drapes close to the body. This is where most supermarket multipacks and fast fashion tees sit. It is cheap to make and cheap to ship, which is most of why it is so common.

150 to 180 GSM. Midweight. The default for a lot of branded tees. Opaque in white, holds up reasonably, not especially structured.

180 to 250 GSM. Heavyweight. Noticeably thicker. Stands away from the body rather than clinging, holds a shape at the shoulders and hem, and does not go translucent. This is the range most "heavyweight" tees actually mean.

250 GSM and up. Properly heavy, closer to a light sweatshirt in hand. Great in autumn, warm in July.

Why it changes how a tee looks

Weight is really about structure. A light fabric follows whatever is under it, so it shows every line of your body and every ripple where it has been folded. A heavy fabric has enough body to hold its own shape, so the sleeve sits where the seam puts it and the hem hangs straight instead of rolling.

That is the whole reason boxy fits are usually cut in heavier cotton. A boxy cut in 140 GSM does not read as boxy. It just reads as baggy.

Weight also changes how a print looks. Ink sitting on a dense, stable fabric stays flat and keeps its edges. On thin jersey the fabric moves underneath the ink every time you stretch it, which is one of several reasons cheap tees crack at the print first.

What GSM does not tell you

A number on its own is not the full story, and it is worth knowing where it stops being useful.

• How the cotton is spun. Combed ringspun cotton is smoother and stronger than carded open-end cotton at exactly the same weight. Ringspun is why two 200 GSM tees can feel completely different.
• How it is knitted. A tight knit wears better and pills less than a loose one, whatever it weighs.
• Whether it has been pre-shrunk. An untreated heavyweight tee can lose a noticeable amount of length on its first hot wash. This is worth more to you than five extra grams.
• Whether it is ring-spun single jersey or a slub. Slub yarns give a deliberately uneven texture that some people love and some read as a fault.

So treat GSM as the first filter rather than the answer. It reliably tells you whether a tee will feel substantial. It does not tell you whether it will still fit after ten washes.

The practical version

If you are comparing two tees and one lists its GSM while the other does not, that itself is information. A shop that is proud of its fabric weight publishes it.

For an everyday tee you want to keep for years rather than a season, anything from about 180 GSM up will feel meaningfully different to a multipack. Above about 240 you are buying something closer to outerwear, which is a real choice rather than a better one.

Whatever weight you land on, the thing that determines how long the print survives is not the fabric. It is how you wash it.

Every Trials tee is cut in heavyweight cotton and screen printed by hand in small runs. Have a look at Time Loop and Golden Hour.