Fabric weight in gsm is the mass of a square metre of fabric, measured by a method such as ASTM D3776. Two fabrics can weigh the same and still feel and look different, because the weight is made up in different ways. The way yarn count, knit density, fibre and finishing combine decides thickness, opacity and drape.

Yarn count and stitch density

A fabric made of finer yarn knitted more tightly can weigh the same as one made of coarser yarn knitted more loosely. The tighter, finer one may feel smoother and thinner but cover better, while the looser one may feel bulkier but show through. Ask the mill for yarn count, knit structure, and stitch density, such as courses and wales per centimetre, and compare bulk and sample.

Fibre and finishing

Cotton quality, fibre length and the amount of brushing, peaching or compacting change how the fabric feels. Finishing can change the thickness without changing the weight much. Mercerising, softeners and heavy compacting all affect hand. Ask for the finishing steps in writing, not only the weight.

Measure thickness

ISO 5084 describes a method for measuring the thickness of textiles under a specified pressure. If you add a thickness figure to the spec alongside gsm, you will catch differences in bulk that weight alone misses. Use the same method on the sample and the bulk.

Check width and relaxation

A fabric that was stretched wide on the finishing frame can have the same weight after relaxation but feel thinner when the sample was relaxed. Compare both after the same wash, for instance one using ISO 6330, and measure weight, width and thickness again.

What to specify

Write gsm with tolerance, yarn count and type, knit structure, stitch density, thickness, finishing steps and finished width. Keep the sealed sample and compare each bulk lot against it by feel and by numbers. Opacity is worth checking directly: hold both fabrics over the same dark and light surface under the same lamp, and photograph them. If the mill changed the yarn source, the spinning method or the finishing route between sample and bulk, treat that as a spec change that needs your written approval, even if the weight is unchanged.