Two ways to write it
The textile test standard, ASTM D3776, calls this “mass per unit area”, traditionally called fabric weight. US shops usually list it in ounces per square yard. Most other places use grams per square metre, the same gsm used for paper.
One ounce is 28.35 g and one square yard is 0.836 m², so 1 oz/yd² = 28.35 ÷ 0.836 = 33.9 gsm.
| oz/yd² | About gsm |
|---|---|
| 3 | 102 |
| 4 | 136 |
| 6 | 203 |
| 8 | 271 |
| 10 | 339 |
To go the other way, divide gsm by 33.9.
Light, medium and heavy
There’s no standard cutoff for “lightweight” or “heavyweight”, and shops draw the lines differently. Compare the numbers, not the labels. A heavier fabric isn’t always thicker or stiffer; weave and fibre matter too.
Per square yard or per linear yard
Some sellers, especially for denim and canvas, give the weight of one yard cut off the bolt at its full width instead, in ounces per linear yard. That number depends on the width. To turn it into oz/yd², divide by the width in inches ÷ 36.
A fabric listed at 10 oz per linear yard on a 60 inch bolt is 10 ÷ (60 ÷ 36) = 6 oz/yd², or about 203 gsm.