Fabric weight (gsm) at a glance
- Lightweight
- About 120 to 170 gsm
- Warm weather and high output
- Midweight
- About 190 to 260 gsm
- The three-season default
- Heavyweight
- 280 gsm and up
- Cold weather, low output
- Unit
- Grams per square meter of fabric
What the number measures
Gsm is grams per square meter: cut a square meter of the cloth, weigh it, and that is the figure. It is measured under ASTM D3776 and it describes the fabric rather than the garment, so it does not change with the size you buy.
Because it is a mass and not a thickness, it captures how dense the knit is as well as how thick. That is what makes it more useful on a base layer than anything else printed on the tag.
The bands, and what each one is for
Lightweight runs roughly 120 to 170 gsm. It is the warm-weather and high-output band: thin enough to wear alone in the heat, and the right call for anyone who runs warm or hikes hard.
Midweight runs roughly 190 to 260 gsm and is the three-season default. It is warm enough to be the only thing under a shell on a cold morning and thin enough not to cook you on a climb. If you are only going to own one, own this one.
Heavyweight starts around 280 gsm and goes up from there. That is winter and low-output territory, and it is genuinely too warm for moving fast: it belongs to cold camps, snowshoeing at a walk, and standing around in conditions where you are not making much heat of your own.
Why gsm beats the brand name
Two tops at the same gsm in the same fiber feel far more alike than two tops from the same brand at different weights. So the order to read a spec sheet in is gsm first, fiber second, everything else after that. It is also the number most often buried in a spec table rather than printed on the front of the listing, which tells you something about how brands would prefer you to shop.
Where gsm misleads
It says nothing about fiber. 200 gsm of merino and 200 gsm of polyester dry at different speeds, behave differently once damp, and smell very differently on day three.
It says nothing about knit structure either. A gridded or waffled fabric traps more air than a flat jersey of the same weight, so it feels warmer and breathes better at the same time. Read gsm as the temperature band and the construction as the fine tuning inside it.
What this means when you're choosing
- A gsm figure at all, since plenty of listings only say lightweight or midweight
- Midweight, around 190 to 260 gsm, if this is your first base layer
- The fiber next to the number, because 200 gsm merino and 200 gsm polyester are different garments
- A grid or waffle knit, which is warmer and more breathable than a flat jersey at the same weight
Fabric weight (gsm) vs Denier
| Fabric weight (gsm) | Denier | |
|---|---|---|
| What it measures | Mass of a square meter of finished cloth | Weight of 9,000 m of a single yarn |
| Unit | Grams per square meter | A number followed by D |
| Where you meet it | Base layers, fleece, shirts | Tent fabric, pack fabric, shells |
| What it predicts | How warm the fabric is | How tough the fabric is |
Which to pick: They answer different questions and neither substitutes for the other. Read gsm when you are choosing warmth and denier when you are choosing durability.
The common mistake
Treating a higher number as a better one
Gsm is a temperature band, not a quality score. A 280 gsm top on a summer climb soaks through in twenty minutes and then chills you the moment you stop, which is the exact failure the layer exists to prevent. Buy for the effort level, not for the coldest morning of the year.
Common questions
What gsm should a hiking base layer be?
Around 150 gsm for warm weather and hard efforts, and 190 to 260 gsm as a three-season default. Above 280 gsm is winter clothing that most people overheat in while walking uphill.
Is 200 gsm merino warmer than 200 gsm polyester?
Slightly, once it is damp, because wool keeps some loft with water in the fiber. Dry and at rest they are close. The bigger differences are drying speed, odour and how long the garment lasts.
Does gsm mean the same thing on socks and fleece?
The unit does, the bands do not. Every category has its own useful range, so a 250 gsm base layer and a 250 gsm fleece are not comparable garments. Compare gsm within one type of item.
Where this is defined
- ASTM D3776/D3776M, mass per unit area (weight) of fabric, ASTM International