GSM (Grams per Square Meter)

GSM is the number that tells you, before you touch a fabric, roughly how it will behave. It is the weight in grams of a piece of that fabric measuring exactly one square metre. A 120 GSM cotton is light enough to see daylight through; a 400 GSM cotton is upholstery. Everything from the drape of a summer shirt to the abrasion life of a workwear trouser traces back to this one figure.

Note that GSM in a textile context has nothing to do with the mobile telephony standard of the same acronym. In mills the term is used exclusively for fabric weight, and it is sometimes written as g/m2 or referred to simply as "fabric weight".

How to calculate GSM

The standard method uses a GSM cutter, a circular die that punches out a disc of exactly 100 square centimetres. You cut the disc, weigh it on a precision balance in grams, and multiply by 100. A disc weighing 1.8 grams therefore indicates a 180 GSM fabric, because one square metre contains one hundred such discs.

Good practice is to cut five discs from different points across the width and along the length of the roll, then average them. Fabric weight is rarely uniform: selvedge areas often run heavier than the centre, and weight drifts along a roll as tension changes during weaving or knitting. A single disc from one corner is a measurement of that corner, not of the roll.

If you know construction rather than having fabric to hand, GSM can be estimated from yarn count and thread density, but the calculation differs for woven and knitted fabrics and carries meaningful error. For commercial acceptance, the cutter method remains the reference.

GSM chart by fabric type

Treat these bands as orientation rather than specification. A 250 GSM fleece and a 250 GSM denim weigh the same and behave nothing alike, because weight says nothing about construction, fibre or finish. GSM tells you how much material is present per unit area; it does not tell you how that material is arranged.

Why GSM is a commercial figure, not just a technical one

Fabric is bought by weight and sold by length, and GSM is the exchange rate between the two. A buyer ordering 10,000 metres of 150 cm wide fabric at 180 GSM is ordering 2,700 kg of material. Quote the same order at 190 GSM and it becomes 2,850 kg — 150 kg of extra yarn that someone has to pay for.

This is why GSM tolerance is negotiated into contracts, typically at plus or minus 5 percent. It is also why GSM drift is a quiet profit leak. A mill consistently running 3 percent over target is giving away 3 percent of its yarn on every order, and because the fabric passes every visual inspection, nobody notices until the yarn reconciliation at year end does not balance.

Why GSM must be tracked per roll, not per SKU

Most generic ERP systems store GSM as a single attribute on the fabric item master: "Cotton Single Jersey, 180 GSM". That model is wrong in an obvious way once you have seen a warehouse. The item master records what was ordered. Individual rolls record what was produced, and they are not the same number.

Rolls from different production runs, different machines, or different yarn lots will vary within tolerance. When a customer specifies a tight tolerance, or when a garment's fabric consumption was costed at exactly 180 GSM, the mill needs to allocate rolls that actually meet the spec — which it can only do if measured GSM lives on the roll record rather than the SKU record.

GSM also feeds three downstream calculations that most factories get wrong when the data is held at SKU level. Fabric costing multiplies GSM by area and yarn rate, so a SKU-level GSM produces a costing that is right on average and wrong on every individual order. The 4-point inspection system applies GSM-adjusted defect thresholds, so grading a heavy roll against a light roll's threshold misclassifies it. And specification matching during order allocation needs the real figure to avoid shipping a roll that will be rejected on receipt.

Vastra ERP stores measured GSM at roll level alongside dye lot, shade coordinates, width and length, so costing, 4-point grading and order allocation all run against the roll that will actually be shipped rather than an averaged item-master value.