GSM to oz: fabric weight conversion chart for clothing

To convert GSM to ounces per square yard, divide the GSM by 33.906. To convert ounces per square yard to GSM, multiply by 33.906. So a 180 GSM t-shirt fabric is 5.31 oz/yd², a 300 GSM fleece is 8.85 oz/yd², and a 14 oz denim is about 475 GSM. The chart below runs from 100 to 600 GSM, and every figure in it was calculated from the exact NIST definitions of the ounce and the yard, not copied from another chart.

What is the formula for GSM to oz?

GSM means grams per square metre. The ounce figure used for fabric in the United States, and on many denim swatch cards, is ounces per square yard. The two are linked by two exact definitions published by the US National Institute of Standards and Technology (NIST): one avoirdupois ounce is exactly 28.349523125 grams, and one yard is exactly 0.9144 metres, which makes one square yard exactly 0.83612736 square metres.

Divide the first by the second and you get the factor: 1 oz/yd² = 33.906 g/m² (33.9057 to four decimal places). NIST’s own conversion table, Appendix B.8 of its Guide to the SI, lists the same factor as 0.03390575 kilograms per square metre for each ounce per square yard. So:

• GSM to oz/yd²: oz = GSM ÷ 33.906
• oz/yd² to GSM: GSM = oz × 33.906
• Quick mental check: divide by 34. For 200 GSM that gives 5.88 instead of 5.90, close enough for conversation but not for a specification.

Some charts online use 33.9, 34 or even 33.91. The differences are small, a few hundredths of an ounce, but they add up when a buyer and a mill each convert with a different factor and then argue about a 2% gap. Put the unit you actually mean in the contract, and convert only for reference.

GSM to oz conversion chart, 100 to 600 GSM

We computed every row by dividing the GSM by 33.9057 and rounding to two decimal places. Use it to read a European or Asian fabric spec in the ounces a US colleague or catalogue uses.

The searches people type most often land here: 140 GSM is 4.13 oz, 180 GSM is 5.31 oz and 200 GSM is 5.90 oz. At the heavy end, 400 GSM is 11.80 oz and 500 GSM is 14.75 oz.

Oz to GSM: the reverse chart for US specs and denim

Going the other way is just as common. A US spec sheet, a blank catalogue or a denim swatch card gives ounces, and a knitting or weaving mill in Bangladesh quotes and tests in grams per square metre. These figures are the ounce value multiplied by 33.9057 and rounded to the nearest gram.

Denim weights are usually quoted in ounces. A 12 oz denim is about 407 GSM and a 14 oz denim about 475 GSM, which is why a jeans fabric feels so different in the hand from even a heavy sweatshirt fleece. If you are developing jeans, our denim manufacturing page explains what to specify beyond the weight.

What do these weights mean for tees, sweatshirts and denim?

A conversion only tells you that two numbers describe the same weight. It does not tell you whether that weight suits your garment. The weight bands for each knit, and why the same GSM can feel different in different yarns, are set out in our fabric and GSM guide for knitwear. This article does not repeat them. Instead, here is how to translate between the two languages in a real conversation:

• A US brief that says “6 oz tee” is asking for about 203 GSM jersey. Quote and test it as 203 GSM finished, not 6 oz.
• A European spec that says “240 GSM heavyweight” is about 7.1 oz. A US buyer comparing it with a catalogue blank should compare 7.1 oz with that blank’s stated weight, and check that both are finished weights.
• A hoodie brief at 10 oz means about 339 GSM fleece. Our hoodie and sweatshirt page lists the fleece and terry weights we work in, in GSM.
• A denim brief at 12 oz means about 407 GSM, measured by the mill on the finished, washed or unwashed fabric you name.

Weight also moves cost in a way you can estimate. For the same construction and yarn, a garment cut from 220 GSM jersey carries 10% more fabric by mass than the same pattern cut from 200 GSM. Going from 280 to 400 GSM fleece adds about 43%. Before you ask for “heavier”, work out what the step is in percentage terms, because that is roughly how much more yarn goes into every piece.

For t-shirts, the weight is only one of several decisions. Our t-shirt manufacturing page covers the jersey options, fits and finishes that go with it.

Square yards, linear yards and other conversion traps

Most arguments about fabric weight are not about arithmetic. They are about two people meaning different things by the same word.

• Per square yard or per linear yard. An ounce figure can describe one square yard of fabric, or one running yard of a roll at its full width. The second number depends on the width of the roll, so a 60-inch fabric and a 72-inch fabric of the same construction give different linear-yard weights. Ask which one a figure is before you convert it.
• Finished or greige. Greige is the fabric straight off the knitting machine or loom, before dyeing and finishing. Its weight can differ a lot from the finished fabric you will wear, as the next sections show.
• Tube or open width. Circular knits come off the machine as a tube. A weight quoted per metre of tube is not a weight per square metre until someone accounts for both layers and the width.
• Fabric weight or garment weight. GSM describes the fabric. A finished hoodie’s weight in grams also includes rib, drawcord, labels and seams, so it is not a check on GSM.

How is GSM measured?

Mass per unit area is measured under standard methods, so that two laboratories weighing the same fabric get the same answer. ISO 3801 specifies how to determine mass per unit length and mass per unit area of woven fabrics conditioned in the standard atmosphere for testing. ASTM D3776, the ASTM method, covers the mass per unit area of most fabrics, with four options: a full roll or piece, a full-width sample, a small swatch and narrow fabrics. ASTM notes that results from a small swatch apply to that sample only, not necessarily to the lot it came from.

On a knit factory floor, the everyday tool is a GSM cutter: a circular blade that cuts a disc of known area, which is then weighed on an electronic balance. A 2023 study of three-thread fleece fabrics by researchers at the Bangladesh University of Textiles in Dhaka describes exactly this set-up, a GSM cutter and electronic balance, run to ASTM D3776. The arithmetic is simple: GSM equals the disc’s mass in grams multiplied by 10,000 and divided by the disc’s area in square centimetres. Because the disc is small, a careful mill cuts several discs across the width, away from the edges, and averages them.

Two practical points follow. Fabric should be conditioned before weighing, because cotton absorbs moisture from the air, and a sample weighed straight from a humid store reads heavier. And the weight should be measured in the state you specify, which is almost always finished.

Why the GSM on your sample may not match the greige

Finishing changes weight, often by more than buyers expect. A 2021 study published in Heliyon, by researchers at Khulna University of Engineering & Technology and the Bangladesh University of Textiles, followed a polyester and cotton three-thread fleece through processing. The greige fabric weighed 264 to 274 GSM. After singeing, heat setting, stentering and compacting it weighed 337 to 342 GSM, against a finished target of 300 GSM. The authors explain why: the fabric narrowed, bringing the courses closer together, and it was overfed by 40% in the stenter and 35% in the compactor.

The lesson for a buyer is not that one figure is right. It is that a weight without a state is incomplete. “300 GSM” should read “300 GSM finished, after washing and compacting”, and the mill should plan its greige weight to arrive there. Brushing a fleece or washing a garment can move the figure again, so if you care about the weight after a garment wash, say so and test after it.

How to write fabric weight into a tech pack

• One unit, stated once. Write the weight in GSM if the mill works in GSM. Add the ounce figure in brackets only as a reference, with the factor you used.
• The state. Finished fabric, and after which finishing steps.
• A tolerance you agree in writing, for example 180 GSM ± 5%, which in ounces is roughly 5.04 to 5.57 oz/yd². There is no universal figure, so agree one before bulk.
• The method. Name ASTM D3776 or ISO 3801, or the method the mill’s laboratory uses, and ask for the result on the fabric test report.
• Where it is checked. Weight is checked on incoming fabric before cutting, alongside shrinkage and shade, which is part of what our quality control covers.

Our free tech pack template has a fabric section with space for the weight, its tolerance and the tests.

What to do next

If you have a spec in ounces and a garment in mind, send both. We will convert the weight, tell you whether the construction you want is usually knitted or woven at that weight in Bangladesh, and send knit-down or fabric swatches so you can feel the difference between, say, 180 and 220 GSM before you commit. Start with the quote request form, and include the ounce figure exactly as you received it, so nothing is lost in translation.