Definition
GSM, grams per square meter, is the standard measure of fabric weight: the higher the number, the heavier and usually thicker the fabric.
Fabric shops list weight as GSM, or g/m², in most of the world, and in ounces per square yard (oz/yd²) in the United States and for denim. To convert, 1 oz/yd² is about 33.9 gsm, so a 12 oz denim weighs around 400 gsm.
Rough ranges:
| Weight | GSM | oz/yd² | Examples |
|---|---|---|---|
| Light | under 150 | under 4.5 | Voile, lawn, chiffon, poplin, light jersey |
| Medium | 150–300 | 4.5–9 | Linen, chambray, jersey, double gauze, French terry |
| Heavy | over 300 | over 9 | Denim, canvas, twill, fleece, wool coating |
GSM tells you weight, not behavior: a heavy viscose can be fluid and a light organza stiff. Read it together with the fiber, the weave and the drape to judge a fabric you cannot touch, and compare it with what a pattern suggests. A summer dress usually wants 100 to 180 gsm, trousers 180 to 300, and bags 300 and above.
Two other figures appear on fabric listings. Momme (mm) is used for silk, and 1 momme is about 4.3 gsm. Weight per linear meter depends on the width of the roll, so it cannot be compared between fabrics of different widths.
More in reading fabric descriptions.