GSM is the number every fabric spec, mill quote and product page seems to hinge on, and the one most often misread. It tells you what a fabric weighs, which shapes how it drapes, how warm it is and what it costs. It does not, on its own, tell you whether the fabric is any good.
This guide explains what GSM means, how it is measured and converted, typical GSM by garment type, and how to use it as a spec in textile manufacturing and sourcing.
What is GSM in fabric?
GSM stands for grams per square meter (g/m2). It is the weight of a piece of fabric measuring one meter by one meter. A 200 GSM jersey weighs 200 grams per square meter; a 400 GSM fleece weighs twice as much. The higher the GSM, the heavier, and usually the denser and thicker, the fabric.
Because it is a weight per unit area, GSM lets you compare any two fabrics on a like-for-like basis, whatever their width or however they are sold. Fabric is bought by the linear meter or yard at a given width, so GSM is a fairer comparison than weight per running length.
Why does fabric weight matter?
Fabric weight is one of the first things a customer feels and one of the biggest drivers of how a garment behaves. It sets the drape, the warmth, the opacity and a large part of the cost, which is why it is specified so early in development.
- Drape and hand: lighter fabrics fall softly and move; heavier ones hold structure. The same pattern in 120 GSM and 300 GSM is effectively two different garments.
- Warmth and season: weight tracks warmth loosely, so GSM is a quick shorthand for whether a fabric suits summer or winter, a tee or a sweatshirt.
- Opacity: a low GSM can be see-through, which matters for white shirts, swimwear and leggings, where a thin fabric fails on coverage however good it looks laid flat.
- Durability and cost: more grams usually means more fiber, which tends to wear longer and cost more, though construction can override both.
How is GSM measured?
GSM is measured by cutting a fabric sample of a known area, conditioning it, and weighing it on a precise scale, then scaling up to one square meter. Labs use a round GSM cutter, typically 100 square centimeters, so the reading multiplied by 100 gives the grams per square meter directly.
- The standards: ISO 3801 covers mass per unit area for woven fabrics, and ASTM D3776 is the common US method; both weigh a conditioned specimen of known area.
- Conditioning matters: fabric absorbs moisture, so it is weighed after conditioning in a standard atmosphere, because a damp sample reads heavier than a dry one.
- Knits and wovens: GSM is the usual weight measure for both, but wovens are also specified by yarn count and thread density, so read GSM alongside construction, not instead of it.
GSM to oz: converting fabric weight
Outside metric markets, fabric weight is often given in ounces per square yard (oz/yd2). To convert, multiply the ounces by 33.9 to get GSM, or divide GSM by 33.9 to get ounces. So a 10 oz denim is about 339 GSM, and a 180 GSM tee is about 5.3 oz.
- Ounces to GSM: multiply by 33.9 (more precisely 33.906).
- GSM to ounces: divide by 33.9, or multiply by 0.0295.
- Watch the unit: ounces per square yard is an area weight; ounces per linear or running yard is not, and cannot be converted to GSM without knowing the fabric width.
GSM by garment type: a reference
There is no single right GSM; it depends on the product. The ranges below are typical starting points, not rules, and they shift with fiber and construction.
- T-shirts (jersey): around 130-150 GSM is lightweight and can be slightly sheer, 150-180 GSM is a standard everyday weight, and 180-220 GSM or more is a premium, structured heavyweight.
- Shirting (poplin, oxford): roughly 100-150 GSM, light enough to drape and breathe.
- Sweatshirts and hoodies (fleece, French terry): about 250-400 GSM, where the weight is the warmth.
- Denim: usually quoted in ounces, about 9 to 14 oz, or roughly 300-475 GSM, from summer-weight to rigid.
- Activewear and leggings: around 150-250 GSM, balancing opacity against stretch and recovery.
- Trousers and chinos (twill): around 200-350 GSM, heavy enough to hold a crease and resist wear.
- Bed linen and sheeting: about 100-160 GSM, where a higher figure is not automatically better.
- Towelling and upholstery: 400 GSM and up, the heavy end of the scale; spec these carefully in the tech pack.
What GSM does and does not tell you
GSM is a weight, not a grade. Two fabrics at the same GSM can feel completely different, because weight says nothing about the yarn, the weave or knit, the finish, or the fiber. A thin, tightly woven high-count cotton can weigh less than a loose, bulky one and still be the better cloth.
- It does signal: heaviness, rough density, likely warmth, and a rough sense of cost, since more grams usually means more fiber.
- It does not signal: quality, thickness, softness, durability or thread count on its own; those come from construction and fiber.
A quick example makes the point. A crisp 250 GSM cotton poplin and a spongy 250 GSM knit weigh exactly the same per square meter, yet one is smooth and structured and the other soft and stretchy. The weight is identical; the fabrics are nothing alike. GSM narrows the field, it does not pick the winner, which is why it is always read together with fiber and construction.
This is why GSM belongs in a spec alongside fiber content, construction and a hand description, not as a lone proxy for quality. Chasing a high GSM for its own sake buys weight, not necessarily a better garment, and adds cost and carbon with it.
Using GSM as a sourcing spec
For a brand, GSM is a control lever, not just a description. It drives fabric cost, garment weight and the feel a customer notices, so it belongs in the specification with a tolerance.
- Spec a target and a tolerance: state the GSM and an allowance, commonly around plus or minus 5%, so a mill delivering lighter than agreed can be caught.
- Remember GSM drives cost: fabric is priced largely by weight, so a small GSM increase multiplied across an order is a real number that sourcing and costing should reflect.
- Verify, do not trust the tag: GSM on a supplier sheet is a claim; confirm it by weighing the bulk fabric, the same way any spec is checked before production ships.
Weight also connects to consumption. A heavier GSM at the same width puts more grams into every garment, so the figure you choose ripples through the cost sheet, the shipping weight, and even the sustainability footprint of the order. That is the difference between treating GSM as a label and treating it as a lever.
GSM is one spec among many: fiber, construction, weight and finish, all connected to cost, season and the customer's hand. Apshan structures that fashion knowledge, cited to source, so a weight decision is informed rather than guessed. Request access or see the plans and pricing.