The unit conversion from gsm to ounces is a common conversion in the clothing industry. Gsm means “grams per square meter,” which is a metric measurement of areal (related to area) density. Converting from metric to imperial units is a common need, and converting the areal density to ounces usually means using an imperial unit for the denominator unit as well, usually square yards. To perform this conversion, it helps to know that there are 28.35 grams per ounce and 0.9144 meters per yard.
Convert the numerator unit of the gsm from grams to ounces by dividing by 28.35.
For example, 100 gsm equals 100 gram/m^2 x (1 ounce/28.35) = 3.53 ounces/m^2. Note that m^2 means “meters squared.”
Convert the denominator unit of the result of Step 1 to meters by multiplying by 0.9144^2, where the caret ^ refers to exponentiation.
For example, 3.53 ounces/m^2 x (0.9144 m/yd)^2 = 2.95 ounces/square-yard. Note that only three significant figures were retained in the end result because the starting density in gsm had only three significant figures.
Convert the denominator to any other unit by dividing twice by how many of the other units there are in a yard.
The total conversion from gsm to ounces per square yard is equivalent to multiplying by the number 0.02949, which you can see by comparing the starting number 100 above to the final result 2.95. See the T-shirt Forum link below for a short conversion chart that covers most T-shirt weights. See the Sail Cloth link below for an online calculator. Click on the menu item “Sailcloth Weight Converter.”
The Sail Cloth converter is a little inaccurate. It measures only 0.02942 ounces/square yard for one gsm. According to Google, the conversion is 0.02949, the same as derived in the steps above.