GSM stands for grams per square metre. It is the weight of one square metre of the fabric, and it is the most useful single number on a t-shirt spec sheet — more useful than the fibre content, and far more useful than the price.
It is also the number most brands leave off. If a listing tells you a tee is "premium cotton" without telling you the weight, that is usually because the weight is not worth mentioning.
The weight bands, and what they feel like
There is no official standard here, but the industry clusters into recognisable bands.
| Weight | Category | What it means in practice |
| 120–140 gsm | Lightweight | Fashion tees, layering pieces, most promotional shirts. Drapes close to the body. Often semi-sheer over darker skin or under bright light. |
| 150–180 gsm | Standard | The default for most high street and athletic tees. Fine for training in. Will soften and thin noticeably inside a year of regular washing. |
| 180–220 gsm | Midweight | Where most "quality basics" sit. Holds shape reasonably. A sensible compromise. |
| 220–280 gsm | Heavyweight | Structured enough to hold a boxy shape off the shoulder rather than clinging. Opaque. This is the band where a tee starts behaving like outerwear. |
| 280 gsm and up | Ultra heavyweight | Very structured, slow to dry, warm. Excellent for a certain look, uncomfortable for actual training. |
What actually changes as the number climbs
Four things, and only one of them is durability.
Opacity. This is the most immediate difference. Lightweight cotton lets light through, especially once it has been stretched across the chest and back a few hundred times. Past roughly 200 gsm the problem disappears.
Structure. A heavier knit resists gravity. It sits away from the body and holds a shoulder line instead of following whatever is underneath it. This is why the boxy silhouette that dominates right now is almost always cut in heavyweight cotton — you cannot get that shape from a 150 gsm jersey, no matter how you cut it.
Recovery. Heavier fabric returns to shape better after being pulled. The collar is where you notice this first, and where a cheap tee dies first.
Longevity. More fibre per square metre means more material to lose before the fabric becomes thin. A 260 gsm tee that has lost 15% of its density is still opaque. A 150 gsm tee that has lost 15% is see-through.
Where GSM stops telling you anything
Weight is not quality on its own, and it is worth being honest about that.
A heavy fabric knitted from short, cheap staple cotton will pill faster than a lighter fabric knitted from combed ring-spun yarn. GSM tells you how much fibre is there. It tells you nothing about the length of that fibre, how it was spun, or how it was finished. A 260 gsm tee made badly is a worse purchase than a 180 gsm tee made well.
Heavier is also not better for every use. If you are running in heat, a 260 gsm cotton tee is the wrong tool — it holds sweat and dries slowly. That is a job for a light technical knit. Heavyweight cotton is for lifting, for cold weather, and for the rest of the day.
And beyond roughly 300 gsm the returns turn negative for most people. The shirt becomes stiff, slow to dry, and warm enough to be a problem indoors.
How to check a claim
If a brand publishes a GSM figure, it is easy to sanity check once the shirt arrives. Weigh it on kitchen scales and compare against a rough expectation: a size medium heavyweight tee at 260 gsm typically lands somewhere around 230–260 grams depending on cut and length. If a shirt sold as heavyweight weighs 150 grams, the number was decorative.
Also watch for the oz/yd² conversion, which US brands often use instead. Divide by roughly 33.9 to convert gsm to ounces per square yard. 260 gsm is about 7.7 oz. If a listing says "7.5 oz heavyweight", that is around 254 gsm and the claim is fair.
What we use, and why
Our tees are 260 gsm pure cotton. That is deliberately at the upper end of heavyweight and short of the ultra band, because 260 is where the fabric is opaque and structured but still wearable under load and in a warm gym.
We publish the figure on every product page, alongside real flat garment measurements in inches and centimetres, so you can check the shirt against one you already own before ordering rather than guessing from a generic size chart.
See the 260 gsm tees · How to measure a t-shirt you already own