Fabric weight is the mass of fabric per unit area. Metric mills and tech packs use grams per square metre, written gsm or g/m². US mills often use ounces per square yard, written oz/yd². NIST lists the conversion factor for one ounce per square yard as 0.03390575 kilograms per square metre, which is 33.90575 grams per square metre. So multiply oz/yd² by about 33.9 to get gsm, and divide gsm by about 33.9 to get oz/yd². An 8 oz/yd² fleece is therefore about 271 gsm.

Check which ounce you are being quoted

Some mills quote ounces per linear yard instead of per square yard. A linear yard of a wider fabric weighs more than a linear yard of a narrower one of the same quality, so a number in ounces per linear yard cannot be converted to gsm without knowing the fabric width. Ask the mill to state the unit in writing, including whether the quoted width is the full roll width or the cuttable width.

Which state of fabric is weighed

Weight changes between greige fabric, finished fabric and washed fabric. Finishing, relaxation and shrinkage can raise or lower the weight per area. Fleece weight also depends on how much the surface was brushed or sheared. Specify that the weight is for finished fabric, and say whether it is before or after a named wash such as ISO 6330, since a fabric that shrinks gets heavier per area.

How it is measured

ASTM D3776 describes methods for measuring mass per unit area of fabric. Ask the mill or your lab which method and conditioning they used, and whether the result is an average of several specimens. A single cut from the edge of the roll is less reliable than several from across the width.

Allow a tolerance

Real rolls vary a little. Write a tolerance into the spec, either as a plus and minus percentage or a range, and tie it to the approved sample. The tolerance is your commercial decision; ask what the mill normally holds and whether it applies per roll or on average.

What to write in the tech pack

Give weight in gsm, add the oz/yd² equivalent in brackets for US suppliers, name the test method, the width basis and the finished-state condition. Check the first bulk delivery by cutting and weighing a few specimens from different rolls.