What Is GSM in Fabric? GSM vs Ounces Explained

What GSM measures

GSM is grams per square meter. Cut a piece of fabric exactly one meter by one meter, weigh it, and the reading in grams is its GSM. Because it describes the cloth rather than a garment, you can use it to compare a tee body, a hoodie body or a roll of jersey on one scale. US blank makers mostly quote the same property in ounces per square yard (oz/yd²), and many spec sheets now give both.

GSM rises when more fiber sits in the same area. That can come from thicker yarn, a tighter knit, a denser construction such as fleece, or extra loops on the back of the fabric. So GSM usually tracks thickness, warmth and opacity. It does not tell you how soft the fabric is, how it drapes, how it fits or how well it takes a print.

How fabric weight is measured

Labs and mills do not cut whole square meters. The usual tool is a round sample cutter that punches a disc of exactly 100 square centimeters, one hundredth of a square meter. The disc goes on a precision scale and the reading is multiplied by 100 to give GSM. Test standards such as ASTM D3776 set out the details, including that fabric is weighed after conditioning in a standard atmosphere. That matters because cotton absorbs moisture from the air: a sample weighed on a humid day reads heavier than the same fabric in a dry room.

Mills knit to a target weight and accept a small tolerance, so a stated weight is a target rather than a promise for every square meter. Some makers quote a range instead of one figure. Shaka Wear, for example, lists its Max Heavyweight tee as 7.5 oz and 245 to 255 GSM.

To check a blank yourself, cut a sample from the body, away from seams and ribbing, measure its area carefully, and weigh it on a scale that reads to a hundredth of a gram. Divide the grams by the area in square meters. It is a destructive test, so use a spare or a misprint.

Converting GSM and ounces

The conversion is fixed: 1 oz/yd² = 33.906 GSM, and 1 GSM = 0.0295 oz/yd². In practice:

• Ounces to GSM: multiply by 33.9.
• GSM to ounces: divide by 33.9.

Going the other way, common metric figures convert like this: 160 GSM is about 4.7 oz, 180 GSM about 5.3 oz, 200 GSM about 5.9 oz, 240 GSM about 7.1 oz, 300 GSM about 8.8 oz and 400 GSM about 11.8 oz.

Makers mix units freely, which is why converting matters. Lane Seven prints both, listing its LS14001 hoodie fleece as 8.25 oz (280 GSM). Blanks by Thirteen quotes grams only: 220 GSM for its tee and 450 GSM for its fleece. Made Blanks uses ounces for most tees and grams for its newest hoodies, such as the 475 GSM Heavy Relaxed Hoodie. Convert everything to one unit before you compare.

Why two "6 oz" shirts can feel different

• Fabric weight is not garment weight. The number describes the cloth. Collars, tape, hems and the size of the shirt change what it weighs on a scale, and a 2XL in a lighter fabric can outweigh a Small in a heavier one.
• Yarn size and knit density. The same weight can come from coarse yarn knit loosely or fine yarn knit tightly. Coarse yarn gives a drier, more textured fabric; fine yarn packed tight feels smoother and looks flatter. Ring-spun vs open-end cotton explains yarn counts.
• Fiber and blend. Cotton, polyester and rayon differ in density and handle, so a blend and a 100% cotton jersey at the same GSM feel different in the hand.
• Finishing. Enzyme washes, softeners, brushing and garment dyeing change the surface and the drape in ways the weight figure does not capture.
• Units and rounding. One maker rounds to a tenth of an ounce, another quotes a range, and some fabric suppliers quote weight per linear yard instead of per square yard.
• Tolerance. Two production lots of the same style can land at slightly different weights within the mill's allowance.

Typical weights by garment

Fleece spreads widely even inside one maker's line. Cotton Heritage lists pullover hoodies at 7, 8.5, 10 and 14 oz, and Lane Seven's Element Hoodie sits at 13 oz (440 GSM). French terry covers much of the same span as fleece, from light loopback used for summer crews and shorts to heavy terry for streetwear hoodies. Because fleece is brushed on the back and terry is not, a fleece and a terry of the same weight feel different: the fleece warmer and loftier, the terry flatter and more breathable. See French terry vs fleece and how to choose a blank hoodie.

What GSM changes for decoration

Fabric weight affects how a garment behaves on press, not just how it feels:

• Curing. Ink cures when the whole ink film reaches temperature. A thick fleece or super-heavy jersey soaks up more heat than a light tee, so the printer may slow the dryer belt or extend heat-press time. Ask whether your heavy blank has been run before.
• Embroidery support. Dense fabric holds stitches without puckering. Light jersey under about 150 GSM needs firm backing and small, open designs; a 300 GSM fleece takes a dense left-chest logo easily.
• Print feel. A heavy plastisol print on a light tee is the stiffest part of the shirt and can pull the fabric. The same deposit on a 240 GSM tee sits flatter and feels more in proportion.
• Show-through. Light fabrics let a dark garment's color and a pale garment's undershirt show through thin ink. Heavier fabric gives the print a more opaque, even background.

None of these rules out a weight; they are things to tell your printer when you send the spec.

Square yards vs linear yards

When you buy fabric by the roll for cut and sew, check which yard a weight refers to. Weight per square yard is comparable across fabrics. Weight per linear yard is the weight of one yard pulled off the roll at whatever width that roll is, so a wider roll weighs more per linear yard even when the fabric is identical. Ask the supplier for GSM or oz/yd² plus the usable width. The fabric guide for cut and sew covers knit constructions in more detail.

Using GSM when you compare blanks

1. Convert every style on your shortlist to one unit.
2. Compare like with like: a cotton jersey against another cotton jersey, not against a tri-blend.
3. Treat small gaps as noise. A few grams either way is normal tolerance; the jump from 150 to 200 GSM is the kind you can feel.
4. Use weight to build the shortlist, then decide by hand. Two blanks with the same GSM can differ more in feel, fit and print surface than two blanks 30 GSM apart.
5. Record the weight with the style and color when you approve a sample, so you can check that a reorder matches. T-shirt weights explained shows how each weight band suits different buyers.

Useful for: clothing brands, creators.