One acronym, two meanings
Type GSM into a search engine and most of what comes back is about mobile phones. GSM, unqualified, usually means Global System for Mobile Communications, and that is a phone standard, not a textile one.
In fabric, GSM means grams per square metre — written g/m², and sometimes called grammage. It answers a narrow question: how much does one square metre of this cloth weigh? Nothing about quality, and nothing about how many yarns are in it.
If a supplier and a buyer mean different things by the same three letters, the order goes wrong quietly. Neither side is wrong; they were answering different questions. The fix is to write the full name next to the number on the order.
What the number tells you
Weight per unit area is the cheapest way to describe a cloth before you touch it. It lets a buyer order a consistent hand and drape from a distance, and it lets a mill quote the same construction twice.
It is also a blunt instrument. GSM says how heavy the cloth is, not how good it is, not how it drapes, and not how it wears. Two fabrics at the same 120 g/m² can behave nothing alike because yarn, weave and finish all differ. Treat GSM as one field among several, not as the verdict.
| Use | Typical g/m² | What you get |
|---|---|---|
| Voile, lining, sheeting | 40–90 | Sheer, soft, moves easily |
| Light plain or twill | 90–140 | Stable, everyday bedding and shirting |
| Midweight upholstery | 140–250 | Holds shape, hides a little more |
| Heavy upholstery, blackout face | 250+ | Firm, opaque, needs a strong base |
These are the ranges our own polyester home-textile programs fall into. They are not a promise that every construction exists at every point inside them.
GSM is not a yarn count
The most common mix-up at the buying desk is GSM against denier. Denier describes how fine a single yarn is — grams per 9,000 metres. GSM describes how heavy a piece of cloth is. A fine denier woven densely can land at the same GSM as a coarse denier woven loosely, and the two will not feel the same.
Denier has its own page. Both numbers belong on the order, but they are answering separate questions.
Greige or finished
The stage matters more than the digit. Greige GSM is the weight off the loom. Finished GSM is the weight after the route you are buying — and dyeing, printing, brushing, coating and softener all move it, usually downward through shrinkage.
Two mills can both quote “90 g/m²” and mean entirely different cloth. Always name the stage. Finished GSM and greige GSM are different numbers covers the gap in detail.
Converting without losing the hand
The arithmetic is trivial; the risk is in rounding. Multiply g/m² by 0.0339 for oz/yd², and oz/yd² by 33.906 to go back. Put one unit on the purchase order and leave the conversion to a tool, so nobody reconciles a 3% discrepancy by eye later.
The GSM tool does the conversion. It does not replace a lab method — to check a sample, cut a known area and weigh it. ISO 3801 covers mass per unit area, and ASTM D3776 is the equivalent US method; comparing methods first saves an argument about numbers later.
If you want the full measurement procedure rather than the unit, how to calculate fabric GSM walks through the cutter method step by step. For the formal definition and the mill-side detail behind it, see the GSM terminology page.