Merino fabric weight is quoted in grams per square metre. It tells you how much fabric is present in a given area — nothing more. A higher number means more wool, more insulation and more bulk, but it is not a temperature rating, and any chart claiming a comfort range for a gsm figure is inventing it. Effort level and layering decide more than the number does. Browse base layers by weight once you know which band you need.
Match the weight to effort, not temperature
The single most useful question is how hard you will be working. Someone skinning uphill at −5°C generates enormous heat and needs a thin layer that moves moisture. Someone on a chairlift at the same temperature is nearly stationary and needs insulation. Same weather, opposite requirements.
| Weight | Effort level | Typical role | Conditions |
|---|---|---|---|
| 150 gsm | High output | Next-to-skin under a shell, or worn alone | Summer, hard aerobic work, travel |
| 190 gsm | Moderate to high | Everyday base layer, standalone top indoors | Three-season, most Canadian winter days under a layer |
| 200 gsm | Moderate | Base layer with a mid-layer over it | Winter commuting, resort skiing, cold hiking |
| 235–250 gsm | Low to moderate | Warm base layer, or a light mid-layer | Cold, low-output days; camp wear |
| 300 gsm | Low | Insulating mid-layer or expedition base | Deep cold with little movement |
The overlap between bands is genuine. If you are between two, the lighter one is usually the more versatile purchase.
150 gsm: high output and warm weather
The lightest common weight. It packs to nothing, dries fastest and layers without bulk under a jacket. It is the correct choice for aerobic effort in the cold, where anything heavier leaves you soaked in your own sweat.
The costs are durability and opacity. At this weight pure merino is fragile — it will pill under a pack strap and can wear through in a season of regular use — and light colours are often semi-transparent. A nylon-blended 150 lasts considerably longer than a pure one.
190 and 200 gsm: the versatile middle
If you buy one merino garment, buy it in this range. It is warm enough to wear under a shell through most of a Canadian winter, breathable enough for moderate effort, substantial enough to wear on its own indoors, and durable enough to survive regular washing.
The difference between 190 and 200 is smaller than the marketing suggests. Fit, knit structure and whether the garment has a collar will each affect warmth more than that 10 gsm gap. Choose on cut and price rather than agonising over the number.
235 to 260 gsm: cold and standing still
These weights prioritise insulation over drying speed. They suit lift-served skiing, ice fishing, winter dog walks, spectating, and work that involves standing outdoors. Worn during hard aerobic effort they will overheat you.
This band also works as a light mid-layer over a thinner base — a more adaptable arrangement than one thick layer, because you can shed half the system when you warm up.
300 gsm: deep cold and low movement
Expedition weight. Genuinely warm, genuinely bulky, and too much for most people most of the time. It earns its place in sustained deep cold with minimal movement, and is otherwise more insulation than a base layer needs.
Before buying at this weight, consider whether two lighter layers would serve better. A 150 base under a 200 mid-layer is warmer than a single 300 in still air, adjusts to changing effort, and is useful across far more of the year.
Knit structure changes what a weight feels like
Two garments at identical gsm can behave quite differently depending on how the yarn is knitted. A single jersey is thin, smooth and fast-drying. An interlock is denser and more opaque at the same mass. A rib traps more air and stretches more. A waffle or grid knit holds loft against the skin while leaving channels for vapour to move.
This is why a 200 gsm waffle can feel warmer than a 250 gsm jersey, and why comparing weights across brands is less precise than the numbers imply. Where a brand publishes the knit as well as the mass, use both. Where it publishes neither, the number on its own is weak information.
Blend matters similarly. A merino-nylon blend at 190 gsm contains less wool than a pure merino garment at the same weight, so it will be slightly less warm and considerably more durable. That trade is usually worth making at high-wear points, and rarely worth making for a garment worn under a jacket.
Why the layering system beats the number
Fabric weight is one variable among several, and rarely the dominant one. A windproof shell over a 150 base layer will keep you warmer in wind than a 250 base layer with nothing over it, because moving air strips heat from a knit far faster than still air does. Fit matters similarly — a garment that gaps at the waist ventilates warmth regardless of its gsm.
Decide the whole system first, then choose weights to fill the roles. Our Canadian winter layering guide covers building that system, and the base layer buying guide covers fit, knit and blend once you have settled on a weight.
Related merino guides
Sources
Image: Merino Mart catalogue. Browse base layers.