What GSM Actually Means When You're Buying a Jacket

Most product pages tell you a jacket is "heavyweight" and leave it there. It is a marketing word. GSM is the number underneath it, and once you can read it you can judge a garment you have never touched.

What the number measures

GSM stands for grams per square metre. Take one square metre of the fabric, put it on a scale, and the weight in grams is the GSM. That is the whole definition.

It is a measure of mass per area, not thickness. That distinction is the reason two fabrics with the same GSM can feel completely different, which we will come back to.

You will sometimes see the imperial equivalent, ounces per square yard. Roughly, 1 oz/yd² ≈ 34 GSM. So a "12 oz" denim or twill is about 400 GSM. Both numbers describe the same thing.

What the ranges actually feel like

Numbers only help if you can map them to something physical. For outerwear and heavier knits:

• 180–280 GSM — shirting, lightweight sweats. Drapes, folds small, no structure.
• 280–320 GSM — mid-weight fleece. Warm, soft, still collapses when you hold it up.
• 350–420 GSM — proper outerwear weight. Holds a shoulder line, stands up on its own, layers over a hoodie.
• 450 GSM and up — heavy workwear and raw denim. Stiff at first, breaks in over months.

For reference, across our own pieces the fleece sits at 280–320 GSM, the sherpa at 380–420 GSM, and the cotton twill bodies at 380–420 GSM, which is about 12 oz per square yard. That twill is the one that gives a jacket a structured shoulder instead of a soft one.

Why two 400 GSM fabrics feel nothing alike

This is where GSM stops being the whole story. Because it measures mass rather than thickness, construction changes everything.

A 400 GSM sherpa is mostly air held in a lofted pile. It is thick, light in the hand, and insulates well. A 400 GSM cotton twill is dense and compact — thinner, but it feels weightier and structured. Identical number, opposite garments.

So read GSM alongside two other things: the fibre (cotton behaves differently from polyester at the same weight) and the construction (woven, knitted, or pile). GSM tells you how much fabric there is. Those tell you how it is arranged.

How to use it when buying

Use GSM to check a claim rather than to pick a winner. If a jacket is described as heavyweight outerwear and the spec says 220 GSM, the description is doing work the fabric is not. If a brand does not publish the figure at all, that is information too.

Higher is not automatically better. A 500 GSM jacket is a genuine commitment to wear in autumn. The right question is not "how heavy" but "heavy enough for what I will actually use it for".