Fabric Weight and GSM Explained: What the Numbers Mean
Learn what GSM and ounces per square yard mean, how fabric weight affects warmth and drape, and typical weights for t-shirts, shirts, denim and suiting.
Key takeaways
- GSM means grams per square metre. A higher number means a heavier, usually thicker fabric.
- Weight affects warmth, drape and durability, but it is not the same as quality.
- To convert ounces per square yard to GSM, multiply by about 33.9.
When a product page lists a fabric as 180 GSM or 12 oz, it is describing weight. This number helps you predict how a garment will feel and perform before you buy it, especially for t-shirts, hoodies, denim and bed linen.
What GSM means
GSM stands for grams per square metre. It measures the weight of a one-metre square of fabric. A lightweight fabric might be around 100 GSM, while a heavy denim can be over 400 GSM. Some brands in the United States and elsewhere use ounces per square yard instead.
Converting between units
One ounce per square yard equals about 33.9 GSM. A 12 ounce denim is therefore roughly 407 GSM. To go the other way, divide the GSM value by 33.9.
Typical weights by garment
| Garment or fabric | Approximate weight | What to expect |
|---|---|---|
| Lightweight t-shirt | Under about 150 GSM | Thin, breathable and good for heat, but may be see-through |
| Midweight t-shirt | About 150 to 200 GSM | A balance of comfort and durability |
| Heavyweight t-shirt | Above about 200 GSM | Thick, structured and long lasting |
| Dress shirt fabric | Roughly 90 to 150 GSM | Light and smooth, with good drape |
| Hoodies and sweatshirts | Roughly 250 to 450 GSM | Warmer as the number rises |
| Denim | Roughly 270 to 680 GSM | Lightweight to very heavy, equal to about 8 to 20 ounces |
| Suiting | Roughly 200 to 350 GSM | Lighter for summer, heavier for winter |
These ranges are guides. Different makers, fibres and fabric types use different conventions.
What weight tells you
- Warmth: heavier fabrics generally trap more air and feel warmer.
- Drape: lighter fabrics tend to hang and flow, while heavier ones hold their shape.
- Opacity: very light fabrics can be see-through, especially in pale colours.
- Durability: heavier fabrics often resist wear better, but fibre quality and construction matter too.
What weight does not tell you
A high GSM does not guarantee quality. A thick fabric made from short, weak fibres can pill and wear out quickly, while a lighter fabric made from long, strong fibres can last for years. GSM is also different from thread count, which counts threads per square inch in woven fabric and is used mainly for bed linen and shirts.
How to use GSM when shopping
- For summer, look for lighter weights in breathable fibres such as cotton and linen.
- For winter, choose heavier weights, and combine them with the layers in our layering guide.
- Compare GSM within the same type of garment, because a 200 GSM t-shirt and a 200 GSM jacket lining are not the same thing.
- Check the fibre as well. Our natural fabrics comparison explains how fibres differ, and our jeans guide covers denim weights in more detail.
