Fabric weight is the specification brands quote most and understand least. It is useful, but it describes only one property.
What GSM measures
Grams per square metre is mass per unit area. It correlates loosely with thickness and opacity, and it is a reasonable shorthand for comparing similar constructions.
What it does not tell you
- Drape, which depends on fibre and yarn as much as weight
- Hand, or how the fabric feels
- Durability, which depends on construction and yarn quality
- Opacity, which varies with fibre and colour
Why identical weights feel different
Two fabrics at the same GSM can behave completely differently depending on fibre content, yarn twist, knit or weave structure and finishing. Weight is a starting filter, not a specification.
Typical ranges as orientation
Lightweight jersey sits well below the weight of a heavy fleece, with midweight tee fabric in between. Treat published ranges as orientation and confirm with a physical swatch, because category names are used loosely across suppliers.
Weight affects cost twice
Heavier fabric costs more per yard and adds shipping weight. It can also be slower to sew. If a design does not need the weight, it is an easy place to reduce cost without reducing quality.