The conversion
One ounce per square yard equals 33.906 grams per square metre.
oz/yd² = GSM ÷ 33.906 and GSM = oz/yd² × 33.906
So a 6 oz t-shirt fabric is about 203 GSM, and a 400 GSM fleece is about 11.8 oz.
Weight per running metre
Add the fabric width to see how much one metre of length weighs. This is how you convert a price per kilogram into a price per metre: a 180 GSM fabric that is 180 cm wide weighs 324 g per running metre, so one kilogram is about 3.09 metres.
Common questions
Is fabric "oz" the same as oz per square yard?
For apparel fabric, yes. A "6 oz tee" means 6 ounces per square yard. Denim is also quoted this way.
Why do US and European suppliers use different units?
US mills traditionally quote ounces per square yard and most other countries use GSM. They describe the same property.