What GSM actually predicts
Mass. Directly, that's the definition. A 180 g/m² jersey and about 1.1 m² of fabric gives a men's M t-shirt weighing roughly 200 grams — which is how you sanity-check a supplier's claim with a kitchen scale.
Opacity, roughly. Within one structure, heavier is less see-through. A white 140 g/m² jersey is translucent and a white 200 isn't.
Shipping and duty cost. Unglamorous and real. Weight per garment times units is a line on your landed cost.
What it does not predict
Drape. Two fabrics at 240 g/m² — an interlock and a french terry — hang completely differently, because drape depends on bending stiffness, and stiffness depends on structure and yarn, not mass. This is the single most common wrong inference from GSM.
Warmth. Insulation is trapped air. A brushed fleece at 320 g/m² is warmer than an unbrushed terry at the same 320, because the pile holds air the loops don't. Same number, different season.
Softness or durability. Both come from the yarn. A combed ring-spun 160 outlasts and out-feels a carded open-end 200. Weight can't see the yarn.
Stretch. That's structure and elastane. A 220 g/m² rib stretches enormously more than a 220 g/m² jersey.
The chart for knit apparel
| g/m² | Reads as | Typical garment | Structure |
|---|---|---|---|
| 110–140 | Very light | Summer tee, layering tank | Single jersey |
| 150–170 | Light | Standard t-shirt | Single jersey |
| 180–190 | Mid | Everyday t-shirt, long sleeve | Single jersey |
| 200–220 | Heavy | Heavyweight or boxy tee | Jersey or interlock |
| 220–260 | Structured | Polo, rib top, structured tee | Interlock, pique, rib |
| 260–300 | Light sweat | Lightweight crewneck | Terry or fleece |
| 300–350 | Sweat | Hoodie, crewneck sweatshirt | Terry or fleece |
| 350–400 | Heavy sweat | Heavyweight hoodie | Terry or fleece |
| 400+ | Very heavy | Specialist streetwear | Fleece |
Two traps in the number itself
Greige versus finished weight. Fabric is heavier before finishing than after, or lighter, depending on the process. A mill quoting 180 may mean either. Ask which, and put it on the tech pack.
Oz/yd² conversions. American suppliers quote ounces per square yard. The conversion is roughly 1 oz/yd² = 34 g/m², so a "6 oz" tee is about 200 g/m² and a "4.5 oz" is about 155. Rounding these in both directions is how a spec drifts by 10 g/m² between two emails.
What to write on the tech pack instead
Weight is one of five fields, not the spec:
- Structure — single jersey, interlock, pique, rib, terry, fleece
- Weight in g/m², stated as finished
- Composition, with elastane percentage
- Yarn — combed or carded, ring-spun or open-end, and the count
- Shrinkage, split into length and width
Get those five and the fabric is pinned down. Get only the second and you've described several hundred fabrics.
Questions
What is GSM in fabric?
Grams per square metre: the mass of a one-metre square of the fabric. A 180 GSM jersey weighs 180 grams per square metre, so a size M t-shirt using about 1.1 m² of it weighs roughly 200 grams.
Is a higher GSM better quality?
No. GSM is mass, not quality. A 220 GSM carded open-end jersey can pill faster and feel rougher than a 160 GSM combed ring-spun one. Weight, yarn and finish are three separate decisions.
What GSM is a good t-shirt?
140 to 160 for lightweight, around 180 for everyday, 200 to 220 for heavyweight. What changes with weight is opacity, how the hem hangs and how much the shoulder seam holds its line.
All five knit structures · jersey weights · terry weights · fleece weights · measuring a finished garment