7 Lightweight Merino Base Layer Bottoms That Thru-Hikers Swear By

It’s 5 a.m. in the High Sierra, and frost glitters on your tent fly. The thought of wriggling out of your warm quilt into the pre-dawn chill is brutal, but you have miles to make. This is where the right base layer bottoms transform from a luxury into a non-negotiable piece of your thru-hiking system. They are your secret weapon for staying warm at camp, sleeping comfortably through plunging temperatures, and even hiking on the coldest days without adding bulk.

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Choosing Your Merino: Weight, Fit, & Durability

When you see numbers like 150 or 200 on a merino product, that’s the fabric weight in grams per square meter (g/m²). Think of it as a warmth rating. A 150-weight is the three-season sweet spot for most thru-hikers—light enough for a pack, warm enough for chilly nights. A 200 or 250-weight is for colder conditions, like starting the Appalachian Trail in February or navigating the Rockies on the Continental Divide Trail.

Fit is crucial for performance. For active use, you want a snug, next-to-skin fit so the wool can effectively wick sweat away from your body. For a piece intended purely for sleeping and camp comfort, a slightly looser fit can feel less restrictive and trap a little more warm air. Decide if this is your sleep system or a potential hiking layer first.

Let’s be honest: ultralight merino wool is not the most durable fabric on the planet. Its fine fibers can be susceptible to abrasion and snags. To combat this, many top brands use a core-spun or blended fabric, wrapping merino wool around a stronger nylon or polyester core. This gives you the best of both worlds: merino’s soft feel and odor resistance with the added strength needed for the rigors of a multi-month trek.

Smartwool Classic 150: The All-Around Thru-Hike Staple

If there is a default, "can’t-go-wrong" option for a long-distance hike, this is it. The Smartwool 150 bottoms have been a cornerstone of hiker kits for years for one simple reason: they just work. They hit the perfect balance of being warm enough for most three-season nights without being so heavy that you resent carrying them through a heatwave.

These are the ultimate multi-taskers. They are your camp pants, your pajamas, and your emergency warmth layer for that unexpected snowstorm in June. The 100% merino construction is exceptionally comfortable against the skin after a long day in synthetic hiking shorts, and their ability to resist odor is a gift on any trip longer than a weekend. For a first-time thru-hiker building a kit, the Smartwool 150 is a safe and reliable starting point.

Icebreaker Oasis 200 for Versatile Cold-Weather Comfort

Stepping up to a 200-weight bottom like the Icebreaker Oasis is a strategic move for hikes with guaranteed cold. Think shoulder seasons, high-elevation routes, or for anyone who simply runs cold. That extra 50 g/m² of wool makes a noticeable difference when the temperature drops below freezing, turning a shivery night into a comfortable one.

While heavier than a 150-weight, the Oasis 200 remains highly packable and serves a dual purpose. It’s an excellent sleep layer but is also streamlined enough to be worn under hiking pants or rain pants during frigid morning ascents. For a Pacific Crest Trail hiker heading into the Sierra or a Colorado Trail hiker expecting frosty nights, the added warmth provides a significant comfort and safety margin for a minimal weight penalty.

Patagonia Capilene Air: Unbeatable Warmth-to-Weight

When every single gram matters but you refuse to compromise on warmth, the Capilene Air bottoms are in a class of their own. Patagonia’s unique 3D-knit structure creates a lofty, airy fabric that traps an incredible amount of heat for its shockingly low weight. They are made from a blend of merino wool and recycled polyester, offering a soft feel with enhanced wicking and dry times.

This performance comes with a critical tradeoff: durability. The open, lofty knit that makes them so warm is also prone to snagging. These are not the bottoms for bushwhacking or lounging around on abrasive granite slabs. Treat them as a specialized tool—a "puffy for your legs"—to be worn primarily for sleeping or under a protective shell. For the dedicated ultralight hiker, their warmth-to-weight ratio is simply unmatched.

Ridge Merino Aspect for Superior Fit and Trail Comfort

Ridge Merino built its reputation on thoughtful design, and the Aspect bottoms showcase that focus perfectly. They utilize a core-spun fabric—a nylon core wrapped in merino—to solve the classic durability problem without sacrificing the feel of wool. This makes them a fantastic choice for hikers who are tough on their gear.

Where the Aspect truly shines is in its fit and construction. Features like a gusseted crotch and flatlock seams, which eliminate chafing, make a huge difference in comfort, whether you’re sleeping or moving. For hikers who find other brands too restrictive or poorly patterned, the Aspect often provides that "just right" fit that moves with you. It’s a workhorse piece designed by people who clearly spend a lot of time on the trail.

Black Diamond Rhythm Tights: The Ultralight Choice

For the minimalist who analyzes every item in their pack, the Black Diamond Rhythm Tights are a compelling option. Built with NuYarn wool technology, which drafts merino fibers around a nylon core, this fabric is lighter, stronger, and stretches more than traditional merino. The result is a base layer that feels almost weightless in your pack and on your body.

These are not the warmest bottoms on this list; their purpose is to provide just enough warmth for the ultralight hiker. They are designed for efficiency, taking the edge off a cold night without a single unnecessary gram. If your goal is to have the lightest functional kit possible for a fast-and-light thru-hike, the Rhythm Tights deliver essential performance in the smallest, lightest package.

ibex Woolies 1: Simple, Dependable Trail Performance

Sometimes, you don’t need fancy blends or complex knits. You just need high-quality, 100% merino wool that is soft, warm, and reliable. The ibex Woolies 1 deliver exactly that. They are a throwback to the simple, effective nature of wool, offering exceptional next-to-skin comfort and the natural performance benefits that made merino famous.

The Woolies are a straightforward, no-frills base layer that excels at its primary job: keeping you comfortable. They are an excellent choice for hikers with sensitive skin or those who prefer the feel of pure merino over blends. Their simplicity is their strength, providing dependable warmth and breathability day after day on the trail.

Minus33 Kancamagus: Durability on a Thru-Hiker Budget

Outfitting a thru-hike is expensive, and premium merino wool often comes with a premium price tag. Minus33 offers a fantastic alternative with its Kancamagus bottoms, providing solid merino performance at a much more accessible price point. This makes them a go-to for hikers on a budget or anyone who sees base layers as consumable items that will inevitably wear out over 2,000 miles.

While they may not be the absolute lightest or feature the most cutting-edge fabric technology, they are durable, warm, and comfortable. They are a true workhorse piece, proven over countless trail miles by hikers who prioritize value and reliability. For a gear list that needs to be both functional and affordable, the Kancamagus is an incredibly smart choice.

Ultimately, the perfect base layer bottom is the one that keeps you warm enough to sleep well and get an early start on the trail. Don’t get paralyzed by the options; consider your typical hiking season, your personal thermostat, and your budget. The goal isn’t to have the most optimized gear, but to have a system that works for you so you can stop worrying about your kit and focus on the incredible journey ahead.