GSM is grams per square metre — the weight of one square metre of the fabric. Cotton underwear generally sits between about 140 and 220. A higher number means more cotton in the same area: more opaque, more durable, slower to dry. A lower number means more airflow and a lighter feel, with less material to wear through.
GSM in underwear gets printed on product pages far more often than it gets explained. It is a useful number, but it is not a quality score, and a heavier fabric is not automatically a better one. What follows is what the number measures, what changes as it rises, and why two fabrics at the same GSM can behave nothing like each other.
The short answer
— GSM is grams per square metre: fabric mass, measured under ISO 3801 or ASTM D3776.
— Within one knit structure, airflow falls steeply as weight rises. One study measured a single jersey at 802 mm/s at 129 g/m² and 305 mm/s at 163 g/m².
— Across different structures the ranking breaks down. A 170 g/m² rib in the same study was nearly four times more permeable than a 163 g/m² jersey.
— More grams means more water held, so slower drying, and generally more material to abrade through.
What does GSM actually measure?
GSM is grams per square metre: the mass of one square metre of fabric. It is measured by cutting a known area, conditioning it and weighing it, under ISO 3801 or ASTM D3776.
That is the whole definition. It tells you how much fibre is present in a given area, and nothing else. It does not tell you which cotton, how it was spun, how it was knitted, or how it was finished — and all four of those change how the fabric behaves on the body.
This matters because GSM is usually the only fabric number a buyer is given. A figure that describes mass ends up being read as a figure that describes quality. They are not the same thing, and the rest of this article is mostly about the gap between them.
What changes between 180 and 220 that you can feel?
Four things move together as weight rises: airflow falls, drying slows, opacity increases, and there is more material to wear through.
Airflow is the one that has been measured most clearly. Selli and Turhan tested commercial cotton knits and found air permeability dropping sharply with mass inside a single structure:
- Single jersey, 30/1 yarn, 129 g/m² — 802 mm/s
- Single jersey, 30/1 yarn, 163 g/m² — 305 mm/s
A 26 per cent increase in fabric mass cut airflow by about 62 per cent. The relationship is not proportional — permeability falls away faster than weight climbs. In practical terms, a step from 180 GSM to 220 GSM costs more ventilation than the size of the step suggests.
Drying follows mass directly: more cotton holds more water, so there is more to evaporate. Weight is the simplest of the variables that decide drying time, and not the only one.
Opacity rises with weight as well, which matters more on light colours than most people expect, and is often the real reason a lighter fabric gets rejected.
Why can two fabrics at the same GSM feel completely different?
Because GSM measures mass and airflow is decided by structure. Change the knit and the weight stops predicting anything.
The same study makes the point plainly. A 2×1 rib at 170 g/m² measured 1,125 mm/s. A single jersey at 163 g/m² — seven grams lighter — measured 305 mm/s. The heavier fabric was close to four times more permeable than the lighter one.
That one comparison is why GSM cannot be read as a quality figure. Knit type, stitch length, yarn count and finishing all move the result, and any of them can overwhelm a forty-gram difference in weight.
It also explains something most people have noticed without being able to name it: two garments both sold as 180 GSM cotton, one of which is clearly cooler to wear. The weight was the same. Everything underneath it was not.
Does a higher GSM last longer?
Usually, but weight is a proxy rather than a cause. What drives abrasion resistance is how tightly the fabric is built, and GSM tends to rise with tightness.
Work on rib cotton knits found weight loss under abrasion increasing as stitch length increased. Longer stitches make a slacker, lighter fabric, and slacker fabric abrades faster. The same study found stitch length had no significant effect on pill formation — a useful separation, because pilling and abrasion are different failures with different causes.
So a heavier fabric generally has more to wear through and is usually built tighter. But a heavy fabric knitted loosely from short-staple yarn will not outlast a lighter one built well. The number is a hint, not a guarantee.
What weight suits an Indian year?
Mid-weight, for most people, most of the time — because the two halves of the year pull in opposite directions and one garment has to serve both.
Through the hot months the argument is for less fabric: more airflow, faster drying, less material holding sweat against the skin.
Through monsoon the argument flips, but not towards weight. Air close to saturated dries fabric slowly whatever it weighs, and a heavy garment that takes a long time to dry becomes a genuine nuisance rather than a minor one.
So neither season argues for a heavy fabric. What the extra weight buys is opacity and working life, and those matter across the whole year. That is what makes mid-weight a considered choice rather than a hedge.
How do you judge fabric weight without the number?
Four checks, none of which needs a lab, and all of which tell you more than a figure on a product page.
- Hold it up to a window. How much light passes through covers both opacity and how open the structure is, in one look.
- Bunch it in your fist and let go. Heavier fabric recovers more slowly and holds the crease a moment longer.
- Feel the edge of a hem or a seam allowance, where two layers meet. Thickness reads more honestly at an edge than across a flat panel.
- Weigh a garment you already own on a kitchen scale. Absolute grams vary with size, but a comparison between two garments in the same size is real.
None of these gives you a GSM figure. They give you the thing the figure was standing in for, which is usually what you wanted in the first place.
Frequently asked questions
What does GSM mean in fabric?
GSM means grams per square metre — the mass of one square metre of the fabric. It is a measure of how much fibre is present in a given area, taken under ISO 3801 or ASTM D3776. It describes weight only, and says nothing about fibre quality, knit structure or finishing.
What is a good GSM for underwear?
Most cotton underwear sits between about 140 and 220 GSM, and mid-weight suits an Indian year best. Lighter fabric ventilates and dries faster; heavier fabric is more opaque and has more material to wear through. Neither end is better in itself — the structure underneath the weight matters more.
Is higher GSM always better?
No. GSM measures mass, not quality. A heavier fabric knitted loosely from short-staple yarn can perform worse than a lighter one built well. In one study a 170 g/m² rib was nearly four times more air-permeable than a 163 g/m² jersey, which shows weight alone does not decide how a fabric behaves.
What GSM is best for hot weather?
Lower weight generally means more airflow and faster drying, both of which help in heat. Measured airflow falls steeply as weight rises — one study recorded 802 mm/s at 129 g/m² against 305 mm/s at 163 g/m² in the same knit. Knit structure changes this as much as weight does.
Does GSM affect how long underwear lasts?
Indirectly. Abrasion resistance is driven by how tightly the fabric is constructed, and weight tends to rise with tightness, so heavier fabric usually lasts longer. Construction is the cause and weight is the proxy, which is why a well-built lighter fabric can outlast a poorly built heavier one.
Why do two fabrics at the same GSM feel different?
Because GSM measures mass and almost everything you feel is decided by structure. Knit type, stitch length, yarn count and finishing all change airflow, surface and drape independently of weight, and any of them can outweigh a forty-gram difference in GSM.
Does GSM change after washing?
Slightly. Cotton knits relax and shrink a little in the first few washes, which pulls the same fibre into a smaller area and raises measured GSM marginally. It is a small effect and not something you would notice wearing the garment.
How is GSM measured?
A specimen of known area is cut, conditioned to standard atmosphere and weighed, then the result is scaled to one square metre. The methods are set out in ISO 3801 and ASTM D3776, which is why the figure is comparable between mills rather than a matter of description.
References
- ISO 3801 — Textiles: Woven fabrics — Determination of mass per unit length and mass per unit area.
- ASTM D3776/D3776M — Standard Test Methods for Mass Per Unit Area (Weight) of Fabric.
- Selli, F. & Turhan, Y. (2017). Investigation of air permeability and moisture management properties of the commercial single jersey and rib knitted fabrics. Tekstil ve Konfeksiyon 27(1), 27–31.
- Shiddique, M.N.A. et al. (2018). Evaluation of Impact of Yarn Count and Stitch Length on Pilling, Abrasion, Shrinkage and Tightness Factor of 1×1 Rib Cotton Knitted Fabrics. Journal of Textile Science & Engineering 8(5), 1000379.