Merino Wool Weight & Temperature Guide: Which gsm Do You Actually Need?

Picking the right merino wool weight is the decision most people skip — and then wonder why their base layer feels too hot in April or too thin in January. The number on the label (150, 170, 200, 250) tells you almost everything you need to know before you buy.

Browse our women's merino wool clothing and you'll see every weight listed clearly — but if you're not sure which one you need yet, this merino wool temperature guide breaks down what each number means, how warm it actually runs, and which weight fits your activity and climate.

What Does the Number in Merino Wool Mean?

The number — 150, 170, 200, 250 — refers to the fabric weight measured in grams per square meter (gsm). It tells you how much the fabric weighs per unit of area.

The higher the number, the heavier and warmer the fabric. The lower the number, the lighter and more breathable it is.

That's it. There's no mystery to it. A 250gsm merino base layer is roughly 67% heavier than a 150gsm T-shirt — and proportionally warmer.

Worth knowing: the gsm number says nothing about quality or fiber fineness. A 150gsm and a 250gsm garment can both be made from the same 18.5 micron merino fiber. The weight only tells you thickness and warmth — not softness or performance grade.

How Warm Is Merino Wool? A Practical Temperature Guide

Merino wool doesn't work like a simple on/off insulator. It regulates temperature in both directions — warming you when it's cold, cooling you when it's hot. The weight you choose determines how strong that effect is.

These ranges overlap by design. A 170gsm shirt worn under a fleece in winter performs very differently than the same shirt on a warm autumn hike. Weight is a starting point, not a fixed rule.

Merino Wool Weight Guide

Choosing the right fabric weight depends on the intended use and the environmental conditions. Here is a detailed guide to common merino wool fabric weights and their suitable uses:

150gsm — Ultralight

150gsm is the lightest practical merino weight. It sits closer to a performance athletic fabric than traditional wool — thin, breathable, and fast-drying.

Use it when the temperature is warm and you're moving. Summer hiking, trail running, cycling, or any activity where heat and sweat are the problem. The 150gsm fabric won't trap warmth the way heavier weights do, which is exactly what you want on a hot day.

It also works well for travel layering — light enough to wear under anything, packable, and it won't add visible bulk under a shirt.

If you tend to run hot or you're in a warm climate year-round, 150gsm covers most of your merino needs.

Explore our women's 150gsm merino T-shirts and men's merino T-shirt collection.

170gsm — Lightweight (The Most Versatile Weight)

170gsm is the weight most people should start with. It's light enough to wear comfortably in mild weather, substantial enough to provide real warmth when temperatures drop, and versatile enough for almost any activity.

This is Merino Protect's most popular weight — and for good reason. A 170gsm T-shirt works for everyday wear, travel, layering, and three-season outdoor activities. It's the one weight that doesn't require you to think too hard about conditions before packing.

If someone asks "which merino should I get first?" — this is the answer. One 170gsm T-shirt can replace three synthetic shirts on a trip and still feel fresh on day four.

It's also the right weight if you're new to merino and not sure where to start. The 150gsm is better if you run hot; the 200gsm is better if you run cold. But most people land comfortably in 170gsm territory.

200gsm — Midweight

200gsm sits in the middle of the range — warmer than the lightweight options, lighter than a true cold-weather layer. It's the most versatile base layer weight.

Use it as a year-round base layer if you live somewhere with genuine temperature variation. It performs from mild autumn days through cold winter mornings without being oppressively warm indoors. Camping, hiking in shoulder seasons, and everyday wear in colder climates all suit 200gsm well.

If you're building a two-piece base layer set, 200gsm top and bottom covers the widest range of conditions before you need to add a mid-layer.

250gsm — Warm

250gsm is Merino Protect's warmest base layer weight. This is purpose-built for cold: skiing, snowboarding, winter hiking, camping below freezing, or anyone who runs cold regardless of season.

The heavier knit traps significantly more air than lighter weights, which is where the insulation comes from. Worn as a base layer under a shell or mid-layer, 250gsm handles genuinely cold conditions — not just "a bit chilly" conditions.

It's also worth knowing that 250gsm, while warm, is still a base layer fabric. It's not an insulating mid-layer or a fleece replacement. If you're in extreme cold, 250gsm base + mid-layer + shell is the correct system.

Explore our men's merino base layer collection and women's merino base layer collection.

What About 320gsm and 400gsm?

You'll see these weights mentioned in comparisons — "250 vs 320 merino wool" is a common search. Here's the honest answer.

320gsm and 400gsm are genuine merino weights, typically used for expedition-level base layers, heavy outdoor shirts, or merino knitwear (sweaters, hoodies). They're warmer than 250gsm and heavier to wear.

Merino Protect's range goes to 250gsm for base layers. For most conditions — including serious winter hiking and skiing — 250gsm is sufficient when layered correctly. The jump to 320gsm+ makes sense primarily for extreme cold environments or extended expeditions where you need maximum insulation from a single layer.

If you're comparing 250 vs 320 for everyday winter use or ski trips, 250gsm with a proper mid-layer will match or outperform 320gsm worn alone — with more flexibility across conditions.

Wool Fabric Weight Chart

Use this chart to match fabric weight, temperature range, and activity at a glance — from ultralight summer layers to warm winter base layers.

Merino Wool Temperature Guide: Which Weight Do You Actually Need?

The honest shortcut:

Run hot, active in warm weather → 150gsm

Want one weight that does most things → 170gsm

Year-round base layer, moderate to cold → 200gsm

Cold climates, winter sports, run cold → 250gsm

If you're still unsure, 170gsm is the right default. It's the weight where most people land after trying a few options — light enough for everyday wear, warm enough to layer in winter.

Ready to Choose Your Weight?

The right merino weight makes a real difference in how a garment performs across seasons. Start with 170gsm if you're not sure — it's the most forgiving weight in the range and the one most customers come back to.

Browse men's merino wool clothing or women's merino wool clothing — every product page lists the weight clearly so you know exactly what you're getting.

All Merino Protect products come with a 90-day warranty. Register your purchase free at merinoprotect.com to extend it to a full year and get 15% off your next order.

Frequently Asked Questions

What does the number in merino wool mean?

The number refers to fabric weight in grams per square meter (gsm). Higher numbers mean heavier, warmer fabric. Lower numbers mean lighter, more breathable fabric. It has nothing to do with fiber quality — a 150gsm and 250gsm garment can both be premium 18.5 micron merino.

How warm is merino wool?

Warmth depends on the weight you choose. 150gsm is suitable for warm weather above 60°F (15°C). 170gsm works in mild to cool conditions around 50–65°F (10–18°C). 200gsm suits temperatures between 40–55°F (5–13°C). 250gsm handles cold below 40°F (5°C) and cold-weather sports when worn as a base layer.

What is the best merino wool weight for hiking?

For three-season hiking, 170gsm is the most versatile choice. For summer hiking in warm conditions, 150gsm. For winter hiking or alpine environments, 200–250gsm as a base layer depending on how cold it gets.

Is 250gsm merino wool warm enough for skiing?

Yes — as a base layer. 250gsm merino worn next to the skin under a mid-layer and ski jacket provides reliable warmth for most ski conditions. Pair with women's merino wool leggings for a complete base layer system. For extreme cold or resort-to-backcountry days, layer accordingly.

What is the warmth rating of merino wool?

Merino wool doesn't have a standardised warmth rating like sleeping bags do. The practical guide is gsm: 150 for warm weather, 170 for mild, 200 for cool, 250 for cold. Layering multiplies the warmth of any single weight significantly.

Can I wear merino wool in summer?

Yes. 150gsm and 170gsm merino are genuine summer fabrics — lighter than many cotton options, better at managing sweat odor, and temperature-regulating. Most people who try merino in summer are surprised by how cool it feels compared to synthetic alternatives.

What's the difference between 250 and 320 merino wool?

320gsm is heavier and warmer than 250gsm, typically used for heavy outdoor shirts or expedition base layers. For most winter activities including skiing, 250gsm layered correctly is sufficient. 320gsm makes sense for extreme cold environments where you need maximum insulation from a single layer.