GSM (Grams per Square Metre)
GSM is the weight of one square metre of fabric in grams. It is the standard way to compare the weight of base layers, fleece and insulation.
For travel base layers, 150 to 200 GSM covers most conditions. Below that is hot-weather territory with reduced durability; above it is cold-weather weight that dries slowly.
Base layer weights
| GSM | Use |
|---|---|
| 120 to 150 | Hot weather, next to skin. Least durable |
| 150 to 200 | The general travel range, roughly 5 to 25 degrees C under layers |
| 200 to 260 | Cool and cold weather, or standalone in mild conditions |
| Above 260 | Cold weather. Too warm and too slow to dry for general travel |
Fleece and mid layers
Fleece is usually described by GSM too. Around 100 GSM is a light grid fleece that breathes well and packs small. Around 200 GSM is the classic mid-weight fleece, the most useful single mid layer for travel. Above 300 GSM you are into heavy fleece that is warm but bulky enough to compete with an insulated jacket for pack space.
Why it beats vague descriptions
Words like lightweight, midweight and expedition weight mean different things at different brands. GSM is a number and it is comparable. If a garment does not publish it, that is worth noticing.
One caveat: GSM compares weight, not warmth. A 200 GSM grid fleece and a 200 GSM flat fleece feel different because construction traps air differently.
Related terms
- Merino vs synthetic base layers: choosing the fibre as well as the weight
- Merino wool: where micron matters alongside GSM
- The practical travel wardrobe: how layers combine
- Glossary: all terms