Fabric weight in grams per square metre always varies a little across a roll and between rolls. Yarn, knitting tension, dyeing and finishing all affect it. A figure such as 180 gsm only means something if you also say how much variation is allowed, how it is measured, and how many samples are taken. Without those, the mill and the buyer can both be right about the same roll.

Weight also drives cost and feel. A few grams below target across a whole lot can save a mill a noticeable amount of yarn, while the garment feels thinner and may shrink or pill differently. That is why the tolerance belongs in the contract and not only on a spec sheet.

Name the method

ASTM D3776 covers measuring mass per unit area of fabric. It includes options for full-width pieces, small specimens and narrow fabrics. Agree which option and which specimen size the mill and your inspector will use. Also agree whether weight is measured on finished fabric before washing or after a named washing procedure such as ISO 6330, since a fabric can change weight once finishing residues and shrinkage take effect.

Set a tolerance

Many buyers express the tolerance as a percentage of the target, for example plus or minus a set percentage, but the right figure depends on the fabric and the product. A lightweight jersey for a premium tee may need a tighter range than a basic fleece. Agree a minimum as well as a maximum; customers notice light fabric more than heavy fabric, and a light lot is also where cost cutting shows up.

Decide where and how often to sample

Agree how many rolls per lot are tested and where on the roll specimens are taken, avoiding the selvedges and the first metres. A sampling scheme such as ISO 2859-1 is designed for attributes in finished goods, so for fabric weight it is usually simpler to agree a fixed number of rolls and specimens per lot.

When bulk comes in light

Ask the mill for the knitting and finishing records of the light rolls and separate them from the rest. Decide in advance whether light rolls are rejected, used for other styles, or accepted at a lower price.

Put the target, tolerance, method and sampling rule in the fabric order, not only in the tech pack, so they travel with the purchase.