If you’ve ever looked at Merino thermals and thought “what in the fleece is 200 GSM?”, you’re not alone. Turns out, it’s all about the weight, and no, not the kind you lift in the gym. 

We’re talking about GSM (grams per square metre), a little number with a big impact on how warm (or not) your Merino wool thermals are.

This guide is here to demystify Merino wool weights and help you find the right fit for your adventures, whether you’re layering up for the slopes or throwing something comfy in your backpacking bag.

Understanding Merino wool weights

Let’s start with the basics. Merino wool weight is measured in GSM, or grams per square metre. It tells you how thick and heavy the fabric is.

Generally, Merino falls into three main categories:

• Lightweight (150-190 GSM): Great for mild weather, layering, or active pursuits

• Midweight (200-250 GSM): The all-rounder. Warm but breathable. It never lets you down

• Heavyweight (260+ GSM): Ideal for freezing temps and low activity (like sleeping in a tent at altitude)

What weight is Merino wool?

Merino clothes and accessories are labelled by their GSM so you can pick based on the climate or activity you’re using it for. 

Lightweight Merino is perfect for high-intensity activities like hiking, trail running, or cycling, where breathability and less overall warmth matter.

Midweight Merino is your more versatile option. Good for cold days but also won’t make you overheat while you’re active.

Heavyweight options are all about insulation. They’re designed for snow trips, base camp lounging or sleepwear when the temps drop below zero.

When someone asks “What weight is Merino wool?”, there’s no one-size-fits-all answer. It all comes down to what you’re doing, where you’re doing it, and how much warmth you need.

What weight Merino baselayer is best for you?

When it comes to picking what weight merino baselayer for you, it really all comes down to where you’re headed, how cold it will be, and what kind of activities you’ll be getting up to.

Lightweight (150-190 GSM): Everyday and active use

This weight is your go-to for layering for shoulder season, warmer climates or high-output activity. Think hiking the Aussie alps in spring, trail running in Tassie, or commuting by bike in the winter. Lightweight Merino keeps you cool, dry, and doesn’t feel bulky under additional layers, like rain jackets or midlayers.

It’s also a top pick for travellers trying to pack light. Lightweight Merino is naturally odour-resistant, breathable, and quick-drying, which means fewer items in your pack and less washing along the way.

Midweight (200-250 GSM): Versatile and warm

If you’re not sure what weight to buy, midweight is a safe bet. You’ll get warmth, but you won’t overheat, you wear it on its own or make it part of your layering system. 

This weight is perfect for snowboarding, skiing, chilly campsites, or winter walks with the dog. It’s also great for those days when the weather can't make up its mind. Cool mornings, sunny arvos, then a drop in temp after dark. Midweight's going to be your best mate.

Heavyweight (260+ GSM): Freezing temps and sleepwear

Heavyweight Merino is all about insulation. Use it when you know you’ll be standing around in extra cold environments or sleeping somewhere frosty. It’s the bulkiest of the three, but the guaranteed warmth makes it a winter staple for serious snow trips.

It’s also a dream for cold sleepers. Chuck it on before bed at altitude or if you’re spending a few nights in the back of a van during winter. Cosy, breathable, and never makes you clammy.

Choosing Merino underwear and thermals by temperature

So, what weight of Merino wool thermal underwear is best for what temperature? A general rule of thumb is;

• Above 10°C: Lightweight (150 GSM)

• -5°C to 10°C: Midweight (200-250 GSM)

• Below -5°C: Heavyweight (260+ GSM)

But remember, this can vary depending on your layering, how much you feel the cold, and what you’re going to be doing.

If you tend to run cold or feel the chill easily, consider stepping up a weight. Likewise, if you’re planning to wear a baselayer under an insulated jacket, or tend to run hot, you might prefer to keep things lighter.

Shop the right Merino weight thermals for you

Hopefully you now have a pretty good idea of what GSM suits you best. No matter if you’re gearing up for an alpine mission, heading off on a multi-day hike, or just trying to stay toasty at your desk without cranking the heater, there’s a Merino weight to suit.

Chasing peaks or trails? Go lightweight. Mixing movement with chill? Midweight’s your mate. Sleeping in a snow cave (or close to it). Heavyweight will be the winner.

What’s more, Merino wool does more than just keep you warm. It regulates body temperature, helps you not to smell, feels great against the skin, and is loaded with sustainable benefits like being biodegradable and renewable. All of which makes it the smart choice for any adventure.

Browse our full range of men’s Merino thermals and women’s Merino thermals to find your perfect fit (added bonus: they’re also Woolmark® and Woolmark Blend® certified).

Or, got a curly question we didn’t answer? Drop us a line, our customer service team is always happy to talk wool.