What Is GSM in Fabric, and How Do You Convert It to Ounces?
GSM is the number most fabric spec sheets lead with, and it is also the number most people are least sure how to read. It stands for grams per square metre: lay a piece of the cloth flat, cut out exactly one square metre, weigh it, and the reading in grams is its GSM. A 150 GSM cotton weighs 150 g per square metre. Nothing more mysterious than that.
The confusion starts when a second unit turns up. North American shops, outdoor-gear makers and almost the entire denim trade quote fabric weight in ounces per square yard, written oz/yd² or just "oz". Both figures describe exactly the same property, the mass of a given area of cloth, so one converts to the other with a fixed factor. This guide explains that factor, shows the conversion on real weights, and covers what GSM can and cannot tell you about a fabric. You can run any of the numbers yourself in the Fabric Weight Converter.
What GSM actually measures
GSM is an areal density: mass per unit area. It says nothing directly about thickness, fibre, or how a fabric feels, only how heavy a fixed patch of it is. That is still a very useful number, because for fabrics of the same fibre and construction, heavier usually means more yarn per square metre: thicker yarns, more of them, or both. Heavier cloth tends to be more opaque, warmer and harder-wearing, and it drapes differently.
Because it is a mass per area, GSM does not depend on the width of the bolt or the length you buy. A 150 cm wide roll and a 112 cm wide roll of the same cloth have the same GSM, even though a metre of the wider one weighs more. That second figure, weight per running metre, is a different number, and it is the one that matters when a fabric is sold by length or shipped by weight. The Fabric Weight from Dimensions tool turns a GSM and a size into that total.
The conversion factor between GSM and oz/yd²
One ounce is 28.349523125 g and one square yard is 0.83612736 m². Divide one by the other and you get the factor the converter uses: 1 oz/yd² is equal to 33.905748 g/m². So:
- to go from oz/yd² to GSM, multiply by 33.905748;
- to go from GSM to oz/yd², divide by 33.905748.
For quick mental arithmetic, "about 34" gets you within a fraction of a percent, which is closer than the tolerance most mills hold on weight anyway. For anything you are writing into a spec or a purchase order, use the full factor, or let the converter do it so you do not round twice.
Worked conversions
These figures were run through the converter itself. The weight band in the last column is the converter's own rough label for each GSM range.
| Entered | GSM | oz/yd² | Converter's band |
|---|---|---|---|
| 60 GSM | 60 | 1.77 | Very light |
| 120 GSM | 120 | 3.54 | Light |
| 150 GSM | 150 | 4.42 | Light |
| 200 GSM | 200 | 5.9 | Medium |
| 280 GSM | 280 | 8.26 | Medium |
| 5 oz | 169.53 | 5 | Light |
| 8 oz | 271.25 | 8 | Medium |
| 12 oz | 406.87 | 12 | Heavy |
Two things are worth noticing. First, the oz figure is always roughly a thirtieth of the GSM figure, so a number in the low single digits is almost certainly ounces and a number in the hundreds is almost certainly GSM. If a listing says "weight: 6" with no unit, it is ounces. Second, the bands are wide. A 120 GSM shirting and a 190 GSM poplin both land in "Light", yet they feel very different in the hand. Treat the band as a first sort, not a description.
What the weight bands mean in practice
The converter's bands are a rough guide rather than a standard, and different mills draw the lines slightly differently. As used on this site:
- Under 100 GSM, very light: chiffon, voile, organza, fine lining. Often sheer, usually needs care in cutting and a fine needle.
- 100 to 200 GSM, light: shirting, poplin, lawn, quilting cotton, most dress cotton. The bulk of what home sewists buy for tops and quilts.
- 200 to 340 GSM, medium: twill, chino, sateen, jersey and sweatshirt fabric, lighter denim and chambray.
- 340 to 500 GSM, heavy: canvas, duck, upholstery cloth, most jeans denim.
- 500 GSM and up, very heavy: heavy canvas, felt, coating wool, heavyweight denim.
Knits complicate the picture a little. A 180 GSM jersey and a 180 GSM woven poplin weigh the same per square metre, but the knit will feel thicker and stretchier because a knitted loop structure traps more air than a tight weave. GSM compares like with like best.
How to measure GSM yourself
You do not need a full square metre. Textile labs cut a small sample of known area, often a 100 cm² circle with a dedicated cutter, and scale up. At home:
- Cut a square exactly 10 cm by 10 cm, which is 0.01 m², with a rotary cutter and a ruler. Cut a few from different parts of the piece, away from the selvedge.
- Weigh each on a scale that reads to 0.1 g or better. Kitchen scales that only read whole grams are too coarse for a sample this small.
- Multiply the average weight by 100. A 10 cm square weighing 3.6 g is a 360 GSM fabric.
You can sanity-check that the other way in the weight-from-dimensions tool: 360 GSM over a piece 0.1 m long and 10 cm wide gives a total of 3.6 g. Weigh fabric as it will be used. A sample straight off the roll may still carry sizing or finish that washes out, and fabric picks up moisture from the air, so a sample weighed on a humid day can read a few percent heavier than the same cloth weighed dry. Labs condition samples at a standard temperature and humidity before weighing for exactly that reason.
Common mistakes when comparing fabric weights
Mixing up oz/yd² with ounces per linear yard
Most ounce weights are per square yard, but some suppliers, especially of canvas and duck in the United States, quote ounces per running yard of a stated width. A "10 oz" figure per linear yard of 60-inch cloth is not the same as 10 oz/yd², because a linear yard of 60-inch cloth covers more than one square yard. If a supplier's figure looks out of line with similar cloth, ask which they mean.
Comparing finished and greige weights
Mills sometimes quote the weight of the cloth as it comes off the loom (greige) rather than after dyeing and finishing. Finishing can add weight (coatings, resins) or remove it (washing out size, brushing), and fabric that shrinks in finishing gains GSM because the same yarn now occupies less area. When two quotes disagree, check whether both are finished weights.
Treating GSM as a quality score
A heavier fabric is not automatically better. A 250 GSM T-shirt jersey will feel substantial, but it will also be warmer and slower to dry than a 160 GSM one, and for a summer shirt that is a drawback. The right weight is the one that suits the job.
What GSM does not tell you
GSM leaves out several things that matter as much as weight:
- Fibre. Wool, cotton and polyester have different densities and behave differently at the same weight.
- Construction. How tightly a woven fabric is set, which you can examine with the Thread Count & Cover Factor Calculator, affects wind resistance, opacity and abrasion far more than a few grams either way.
- Yarn fineness. Two 150 GSM fabrics can be made of many fine yarns or fewer coarse ones. The yarn count converter helps you read those specs.
- Behaviour in the wash. Shrinkage changes the finished size, and with it the GSM, of many natural-fibre fabrics. Test it with the Fabric Shrinkage Calculator.
Where the converter stops
The converter does one job: it moves a weight between GSM and oz/yd² using the exact factor and attaches a rough band. It does not know the fibre or the construction, it cannot account for moisture or finishing, and its bands are approximate labels, not a classification any standard body uses. For anything that must meet a specification, such as a uniform contract, a technical textile, or a regulated product, weigh conditioned samples to the relevant test method or ask an accredited textile laboratory. For everyday buying and comparing, the conversion is exact, and that alone removes most of the guesswork.