Woman wearing merino base layer deciding if she want to take her merino mid layer

The weighting is the hardest part

Stuck on which merino weight is right? A beginner's guide to merino GSMs and when to use them.

BY: MARK COHEN • tech corner • updated on 24.04.2024

Close your eyes and think of your perfect trail run. Mine is sometime in late April. It’s dusk. I’m in a car park – maybe near Alpthal in Switzerland. The sun is fading. Conditions are cool but not cold. My headlamp is on and the first 3 km is on light gravel before the trail spikes for a couple hundred vertical metres revealing sunset over Grosser Mythen. I have the next 90 minutes to think about nothing other than putting one foot in front of the other. Bliss.

Now – eyes still shut – put yourself in this moment. What are you wearing on this run? The temp is hovering around 6 degrees, and, in all seasons, you lean towards natural performance fabrics. Do you start with something lightweight and next-to-skin and build from there? An X-Light synthetic base layer, for example, and then a heavier weight merino as a mid layer over top, say 200 GSMs? Lighter? Heavier?

Merino has enjoyed wunderkind status in the outdoors for years owing to its versatility in environments like these. Blended or pure, on trails, running, skiing and daily wear, it boasts infinite uses. But it being wool, it also comes in different weights, each suited to activity type and temperatures. So, which is right for when? And more specifically, which is right for you?  

What’s a GSM?

Merino wools are measured in something called grams per square metre or GSMs. Broken down simply, GSMs indicate the weight of a merino garment. Lighter weight GSMs are made for warmer weather (spring, summer and autumn) whereas heavier weight GSMs are created for colder climes, winter and working outside. So, the heavier the weave or bigger the number, the warmer the piece and vice-versa.

For winter, when people tend to look to wool for comfort and performance in GSMs of 200 or more. In summer, 130 weight weaves are the norm. Light, airy, breathable. Pure and natural comfort. So how do you choose?    

Merino 115-160 GSM

In the world of wool, this is considered lightweight, airy even. Think everyday shirt or boxer shorts, lightweight, soft-touch kind of clothing. Depending on the colour, the wool weave might even be a little translucent but certainly not see-through.

The ideal use cases for lightweight merino are manifold: as an everyday tee (depending on fit), as a base layer for hiking, even as a running top or base layer (depending on preference for natural or performance synthetics and perhaps your own internal thermostat). Wool doesn’t wick moisture as fast as a synthetic, but it breathes and allows vapour to escape - simply at its own pace and with its own dispersion characteristics. The real beauty of these weights is their versatility. One day you’re wearing it at the office, the next, you’re out running uphill in it. Hang to dry and repeat. You’ll be amazed.

Here’s an example of a lightweight merino blend. From running to hiking to just living, you can use this weight a lot. 

Merino 190-220 GSM

Ok, so here we have what is considered middle weight merino. Warm enough to ski and snowboard in. It’s a bit thicker to the touch, still slightly translucent (but barely), a bit warmer on your skin, yet still cool enough for everyday winter wear if you’re simply seeking long underwear as the mercury plumets. Something to insulate and keep you warm.

On the trail run in Alpthal – the dream scenario – this is probably the layer I’d reach for. Thick enough to keep me warm, still breathable enough for heart rates hovering around 160 beats per minute. With a windbreaking layer overtop, I could take this layering combination another ten degrees colder and still be very comfortable. At the end, I’d hang dry, and the layer is ready to roll for another day of play outside. As a comparable, heavyweight cotton tees probably boast the same GSM but would obviously have much different performance characteristics. Here’s an example of a 200 GSM merino base layer set:

Merino 260 GSM and beyond

If you run cold, work outside or simply appreciate the warmth, comfort and natural performance of heavyweight merino, this is the weight you want. It will generate ample warmth when sitting still, but still be every bit as performance ready should the pace pick up (to a point – some may find this simply too hot for ski touring or running, for example). Still naturally thermoregulating, still soft-touch and anti-odour, still breathable, but a noticeably thicker feel and warmer than the previously mentioned weights.

This wool weight is great for cold ski days, windy winter hikes or just sitting tight in front of a warm fire. You can pair this with another layer underneath or wear it next-to-skin – but the net result will be the same. Warmth and comfort.  

Bound only by imagination

Whatever you’re wearing in the mountains or elsewhere, experimenting with merino is a gamble of the possible. If you ride outside all winter, you can wear it. If you run in the heat, you can wear it. If you ski 30 days a year, yup, you can use merino here too. Bound only by imagination, it boasts a certain panache. A certain level of refined performance that’s made me a convert for about two decades with no finish line in sight.


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