GSM shows up on fabric spec sheets and increasingly on product pages, and it is one of the more useful numbers a shirt can tell you before you buy it. Here is what it measures and how to actually use it.
The definition
GSM stands for grams per square meter. It is the weight of the fabric measured over a fixed area, which makes it a direct measure of fabric density and thickness, independent of the garment's size. A higher GSM means a heavier, more substantial fabric. A lower GSM means a lighter, thinner one. It is the same idea as thread count for sheets, a single number that tells you a lot about how the material will feel and behave.
What different weights feel like
- Under 150 GSM: Very lightweight and sheer at the low end. Common in ultra-breathable summer performance wear.
- 150 to 200 GSM: A common range for everyday tees, including most premium bamboo and merino blend shirts. Substantial enough to hold shape and drape well, still breathable enough for training.
- 200 to 250 GSM: Midweight, closer to a structured tee or light sweatshirt fabric.
- Above 280 GSM: Heavyweight, moving into sweatshirt and outerwear territory.
Our merino and lyocell blend runs at 180 GSM, deliberately chosen as a substantial weight that drapes cleanly and holds its shape without tipping into bulky. Full fabric details are in what is a merino and TENCEL blend.
Why GSM matters more than fiber alone
Two shirts in the same fabric can feel completely different depending on GSM. A 130 GSM bamboo tee will feel closer to a lightweight base layer, while a 200 GSM bamboo tee reads more like a structured everyday tee. Neither is better in the abstract, they are built for different jobs. This is why fabric composition and GSM are both worth checking, not just one or the other.
GSM and durability
Heavier fabric generally holds up to repeated wear and washing better than very lightweight fabric, since there is simply more material to withstand friction and stretch. This is one reason a 180 GSM merino blend feels like it is built to last, compared to an ultralight 120 GSM travel tee designed to pack small rather than take a beating.
GSM and season
Lighter GSM breathes more and suits hot weather or high-output training. Heavier GSM holds more warmth and structure, better suited to cooler months or lower-intensity wear. This is part of the practical difference between our featherweight bamboo tees and the more substantial merino blend line; see merino wool in summer for how weight and fiber interact across seasons.
Frequently asked questions
Is higher GSM always better? No. It depends on use. A high GSM summer tee would overheat you. GSM should match the fiber and the intended use, not be maximized on its own.
How do I find a shirt's GSM? Reputable brands list it on the product page or spec sheet alongside fiber composition. If it is missing, that is worth asking about directly.
What GSM is best for athletic wear? Most performance tees fall between 130 and 200 GSM, balancing breathability with enough structure to hold shape through a workout and a wash cycle.
See GSM in context on every Kane Merino Collection product page, alongside full fiber composition and care details.