The weight of a fabric depends of the thickness of the threads it’s made of, of the density of the weave or knit, as well as its composition (linen for instance is 20% heavier than silk). The dyeing or printing process can also affect the weight. Generally speaking, the lighter a fabric, the more drapey it will be, and the heavier, the more opaque and resistant it will be… but that’s not always true! A cotton poplin for instance will be lightweight (≈ 170 g/m²) but stiff, while a thick wool crepe will be heavy (≈ 300 g/m²) but supple.
Therefore it is particularly important when choosing fabric for a project to consider all its characteristics: its weight, but also its drape, stretch, breathability, softness, etc. With time and experience you’ll learn about different types of fabric and you’ll be able to follow your intuition when picking one, whether online or in stores. In the meantime, here is a little guide to help you decrypt this mysterious property that is fabric weight!
Cuts of fabric can have different dimensions, which is why fabric weight is measured in terms of mass per surface area, i.e. how much a given quantity of that fabric will weigh. Depending of the system (metric or imperial), the weight will be measured in grams per square meter (g/m² or gsm) or in ounce per square yard (oz/y², often abbreviated to oz.).
To convert from metric to imperial, you just have to divide the weight in g/m² by a factor of 33.906. For this wool Melton at 340 g/m² for instance:
340 / 33.906 = 10 oz.
Inversely, you can get the weight in g/m² from the weight in oz. by multiplicating by 33.906. For instance, for this 6.5 oz. denim:
6.5 * 33.906 = 220 g/m²
On some websites, you can find the weight given not in g/m² but in g/m or glm, which is grams per linear meter. Attention: this is not equivalent to the weight in g/m²! This is the weight of a cut of one meter of this fabric, so you have to take its width into account.
To find the weight in g/m² from the weight in glm, you simply have to divide the latter by the width of the fabric in meters, for instance for this wool coating with a width of 140 cm and a weight of 476 glm:
476 / 1.40 = 340 g/m²
The distinction between lightweight and very lightweight, or medium or heavy is very subjective. That said, many fabric websites have a relatively standard scale that categorizes fabrics in different weight classes. Here are the ones you’ll find most often:
Remember that the weight of a fabric is only one characteristic among others, and that sewing is not an exact science. If your pattern recommends a medium-weight fabric, and the one that you have your eye on seems “light-medium” rather than “medium-medium”, don’t fret! These indications are there to give you an order of magnitude, but a difference of 10 or 20 g/m² won’t have any impact on your final result.
To make buying fabrics online easier, we recommend you make a swatch notebook: for each fabric, write down its composition and its weight when you buy it. This will give you a whole rainbow of references to compare with the description of a fabric online, so that you can get a better idea of how it’ll be like before you click on “add to cart”!