Walk into any fabric store and you'll see bolts labeled with unfamiliar numbers: 4 oz, 10 oz, 200 GSM. Those numbers describe fabric weight, and understanding them is one of the most practical skills a maker can develop. Weight affects everything from how a bag holds its shape to how comfortable a strap feels on your shoulder.
What Is GSM?
GSM stands for grams per square meter, and it's the most universal way to measure fabric weight. A lightweight cotton might come in at 100 GSM, while heavy denim could weigh 400 GSM or more. In the United States, you'll also see fabric measured in ounces per square yard. The conversion is roughly: one ounce per square yard equals about 34 GSM. So a 10 oz canvas is approximately 340 GSM.
For bag making, you generally want fabrics in the 200 to 500 GSM range. Below 200 GSM, fabrics are too light for structural use unless you add interfacing. Above 500 GSM, you're into territory where most home sewing machines struggle to punch through multiple layers.
Understanding Drape
Drape describes how a fabric falls and moves when it hangs freely. A fabric with lots of drape flows and pools, like silk or rayon. A fabric with little drape holds its shape and stands up on its own, like heavy canvas or buckram. Drape is influenced by weight, fiber content, and weave structure.
For bags and pouches, you generally want low to medium drape. A tote bag needs enough stiffness to stay open when you set it down but enough flexibility to fold flat when it's empty. A pouch needs to hold its shape in your hand but conform to your pocket or the inside of a larger bag. Getting this balance right is largely about choosing the right fabric weight.
Matching Weight to Function
Different parts of a bag call for different weights. The exterior panels need to be heavy enough to hold shape and resist abrasion, typically 8 to 14 oz canvas or denim. Straps and handles should be sturdy but not so heavy that they're uncomfortable, usually the same weight as the body or slightly lighter with webbing reinforcement. Linings should be lighter, 4 to 6 oz, so they don't add unnecessary bulk.
When mixing weights in patchwork, use interfacing to bring lighter fabrics up to the weight of their heavier neighbors. Iron-on woven interfacing is the easiest option. It adds structure without changing the fabric's surface texture, and it prevents lighter panels from stretching or distorting relative to the heavier ones around them.
The Feel Factor
Numbers only tell part of the story. Two fabrics with the same GSM can feel completely different depending on their fiber content and weave. A 300 GSM sherpa feels soft and plush, while a 300 GSM canvas feels crisp and sturdy. Always handle fabric before buying if you can. Rub it between your fingers, drape it over your hand, and fold it to check for stiffness. The way a fabric feels in your hands is the best predictor of how it'll perform in your finished piece.