Field Guide

Fabric weight: GSM, ounces, and what they mean

Fabric weight is the most useful number on a bolt you cannot touch. It tells you, before anything else, whether a cloth will float, hang, or stand on its own, and it is the figure most often missing from an online listing. Weight is quoted two ways: grams per square meter, written GSM or g/m2, which is the international standard, and ounces per square yard, oz/yd2, still common in North America.

Both measure the same thing, the weight of a fixed area of cloth, and converting between them is a single multiplication. Weight is not quality and it is not thickness: a dense, fine silk can outweigh a loose, bulky knit. But read alongside the weave and the fiber, it is the fastest way to predict how a cloth will behave.

Quick conversion table

The common values, rounded. Read either column and cross to the other.

Weight classes, from voile to canvas

Mills and pattern makers sort cloth into rough bands. The lines are not fixed, and different suppliers draw them differently, but these ranges hold for most cloth. Each example links to its full entry in the catalogue.

How to measure GSM without a label

You do not need the bolt end or the spec sheet. With a ruler and a kitchen scale you can read the weight of any cloth you can cut a clean square from.

1. 1Cut a 10 by 10 cm square. Cut a precise 10 by 10 centimeter square of relaxed, unstretched fabric. That square is exactly one hundredth of a square meter, which is what makes the rest of the math trivial.
2. 2Weigh it in grams. Weigh the square on a digital kitchen or jewelry scale. A scale that reads to a tenth of a gram is worth using for light cloth, where a whole gram is a large share of the weight.
3. 3Multiply by 100. Multiply the weight in grams by 100. A square that weighs 1.5 g is 150 g/m2. That number is your GSM, no label required.
4. 4Convert to ounces if you need to. To read it in ounces per square yard, divide the GSM by 33.9. A 150 g/m2 cloth is about 4.4 oz/yd2.

Why weight matters more than it looks

A pattern is drafted with a weight of cloth in mind. A drapey blouse cut in heavy denim stands away from the body and reads like cardboard; a structured trouser cut in a feather voile collapses. The weight band is the first thing to match between a pattern and a bolt, ahead of color and even fiber.

It is also the honest number that a listing photo cannot fake. A cloth can be styled to look substantial; its GSM cannot. When a seller gives a weight, you can predict the hand before it arrives. When a seller omits it, that silence is itself worth reading.

What weight does not tell you

Weight is one axis, not the whole cloth. Two fabrics at the same GSM can behave nothing alike: a tightly woven 200 g/m2 poplin is crisp and stable, while a 200 g/m2 jersey is soft and stretchy, because weave and fiber decide the rest. Weight predicts heft and opacity reliably, drape only loosely, and quality not at all. Read it together with the structure, never on its own.

Common questions