The conversion, and the number to remember
One ounce per square yard equals 33.906 grams per square metre. So gsm = oz × 33.906, and oz = gsm ÷ 33.906.
The three numbers worth memorising: 5.5oz is about 186gsm, 6.1oz is about 207gsm, and 200gsm is about 5.9oz. Those cover most of the retail t-shirt range.
- 4.3 oz ≈ 146 gsm — light promotional
- 5.3 oz ≈ 180 gsm — standard retail light
- 5.5 oz ≈ 186 gsm — the US "standard tee"
- 6.1 oz ≈ 207 gsm — retail heavy
- 7.1 oz ≈ 241 gsm — heavy / streetwear
- 8.5 oz ≈ 288 gsm — statement weight
Why the US quotes ounces and Europe quotes gsm
The US apparel trade has used ounces per square yard for jersey since the mid-twentieth century, and the number is embedded in buyer language: a "5.5 ounce tee" is a recognised product class there in a way "186gsm" is not.
Europe and Asia quote gsm because it is metric and because the fabric mills that supply them weigh in grams. Neither is more accurate; the convention simply follows the market.
The three errors that make a weight "wrong"
Weighing the finished garment instead of the fabric. A shirt weighs its fabric plus trims, so the same fabric produces different shirt weights at different sizes. Gsm is a fabric property; shirt weight is not.
Measuring after washing. A cotton jersey relaxes and shrinks on the first wash, which raises the gsm of the remaining fabric. A 180gsm shirt measured after a hot wash can read as 190gsm and it is not a heavier shirt.
Quoting the greige weight instead of the finished weight. Finishing, compacting and pre-shrinking all change the weight, and the difference between greige and finished can be several percent. A supplier quoting greige is quoting a number the customer will never hold.
How to check a claim without a lab
Cut a 10cm × 10cm square from a flat area away from seams and hem, weigh it on a kitchen scale that reads to 0.1g, and multiply by 100. A 100cm² piece of 200gsm fabric should weigh 2.0g.
That is not a substitute for a lab test, but it is enough to tell a 180gsm shirt from a 230gsm one, which is the question buyers actually have.
What the weight does not tell you
Gsm says nothing about yarn quality, knit tightness, or whether the fabric was compacted. Two 200gsm jerseys can differ in hand, drape and shrinkage — the weight is one number in a spec sheet, and it is the easiest one to quote.
One multiplication, three ways to get it wrong. The arithmetic is not the problem — knowing what was weighed is.