Trail Gear Picks
Buying guide

The Best Base Layers

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Six base layers and long underwear sets for hiking in the cold, merino wool and synthetic, from about $10 to $150. Tops and bottoms, women's cuts, and the weight to buy for your weather.

By Alex HartDay trails mostly, a few nights out a year, mountains when the time allows.
Updated Nov 2026·27 compared

The Smartwool Classic Thermal Merino Crew is the base layer I'd buy for hiking in the cold. It's all merino wool, warm on a winter summit, and it stays warm after you've sweated on the climb. Cotton goes cold and clammy. The top costs about $125 and fits close without squeezing, and matching bottoms cost about as much again.

The Merino.tech set gives you a merino top and long johns together for about $100, the value buy here. Hiking more in fall and spring than in deep winter? The thinner All-Season is the one to wear on the move. The Under Armour ColdGear, about $40, is the synthetic: it dries fast and doesn't mind the washing machine.

See the 6 picks

Our picks

Smartwool Classic Thermal Merino Crew
Our pick
Smartwool Classic Thermal Merino Crew

All merino at 250 grams per square meter, warm enough for a winter day and still breathable on the climb. About $125, with matching bottoms and a women's cut.

Check price on Amazon$125.00
Smartwool Classic All-Season Merino Long Sleeve
Lightweight pick
Smartwool Classic All-Season Merino Long Sleeve

The thin merino top you can hike in from fall to spring and sleep in all summer. About 6.5 ounces and about $100.

Check price on Amazon$100.00
Merino.tech Midweight 250 Base Layer Set
Budget merino
Merino.tech Midweight 250 Base Layer Set

A merino top and long johns in the same weight as the Smartwool, for about what Smartwool charges for the top alone.

Check price on Amazon$89.99
Minus33 Expedition Weight Crew
Warmest pick
Minus33 Expedition Weight Crew

Thick 400-gram merino for ice fishing, hunting stands and long stops in deep cold. About $150, and you can put it in the dryer.

Check price on Amazon$149.99
Under Armour ColdGear Armour Fitted Crew
Synthetic pick
Under Armour ColdGear Armour Fitted Crew

A brushed, stretchy synthetic that dries fast after a sweaty climb, with mesh panels in the back. About $40.

Check price on Amazon$50.22
32 Degrees Heat Lightweight Crew
Budget pick
32 Degrees Heat Lightweight Crew

A thin, stretchy thermal for about $10. Not the warmest, but warm enough when you're moving, and cheap enough to buy two.

Check price on Amazon$9.99

At a glance

Product names go to Amazon. Rank numbers jump to the full write-up.

Base layerBest forFabricWarmthAbout
1Smartwool Classic Thermal Merino CrewCold-weather hiking100% merinoMidweight, 250 g/m²$125
2Smartwool Classic All-Season Merino Long SleeveHiking, three seasons88% merino, 12% nylonLightweight, 150 g/m²$100
3Merino.tech Midweight 250 Base Layer SetTop and bottoms100% merinoMidweight, 250 g/m²$100
4Minus33 Expedition Weight CrewExtreme cold100% merinoHeavyweight, 400 g/m²$150
5Under Armour ColdGear Armour Fitted CrewSynthetic under $5087% polyester, 13% elastaneMidweight, brushed inside$40
632 Degrees Heat Lightweight CrewUnder $15Polyester, acrylic, rayon blendLightweight$10

The best base layers and long underwear, ranked

1Cold-weather hiking
Smartwool Classic Thermal Merino Crew

Smartwool Classic Thermal Merino Crew

All merino at 250 grams per square meter, warm enough for a winter day and still breathable on the climb. About $125, with matching bottoms and a women's cut.

$125.00
Check price on Amazon
100% merino woolInterlock knitFlatlock seamsAbout 10 oz
+Read the full write-up

Why I picked it

The fabric is 250 grams per square meter of merino, the midweight that works from a frosty October morning to a January day below freezing. Wool keeps you warm even damp, it breathes when you push uphill, and it doesn't stink after a weekend. The seams are flat, so nothing rubs under pack straps, and the top weighs about 10 ounces in a men's medium.

It's cut slim, so it sits close to your skin, where a base layer does its job. Matching bottoms turn it into a set of long underwear, and the women's version is sold in the same fabric.

Who it is for

Day hikers and backpackers who go out from late fall to early spring, and anyone who wants one warm wool layer for hiking, camp and skiing.

What owners report

Warmth is what owners mention most. A snowboarder wore his every day for five days in Vail, in weather from 5°F to 55°F, and says it never started to smell. The unhappy review comes from a man who wore it on a brisk five-mile walk at 30 to 40°F, under a sweater and a jacket: it came home soaked with sweat. He now saves it for slower days.

Durability is where buyers part ways. A man who has bought about thirty of them over the years says some last a long time and some fray at the collar sooner than expected, and another owner wears through the elbows. On size, the sleeves run a little long and the fabric loosens after the first wear: a buyer at 6'2" found the large tight at first, then just right.

Flaws but not dealbreakers

  • Merino wears faster than polyester, especially where pack straps rub.
  • Too warm for a steep climb in mild weather. Take the All-Season for that.
  • About $125 for the top alone, and the bottoms cost about the same again.

Fit and sizing

Slim fit in your usual size. It loosens a little after the first wear.

2
Smartwool Classic All-Season Merino Long Sleeve
Hiking, three seasons

Smartwool Classic All-Season Merino Long Sleeve

The thin merino top you can hike in from fall to spring and sleep in all summer. About 6.5 ounces and about $100.

$100.00
See it on Amazon
Merino over a nylon coreWool next to skinSlim fitAbout 6.5 oz
+Read the full write-up

Why I picked it

A lighter top you can hike in for most of the year: 150 grams per square meter, about 6.5 ounces in a medium. The yarn is merino wrapped around a thin nylon core, so only wool touches your skin and the nylon helps it last. It's Treeline Review's best overall, and the only wool layer in their test that showed no wear after four months of hard use.

It suits a cool fall hike, goes under a fleece on a cold one, and works as a sleeping top on a summer night in the mountains.

Who it is for

Hikers who go out in three seasons and run warm on the climbs, and backpackers who want one top to hike and sleep in.

What owners report

One owner bought his fifteen years ago, still wears it, and has just bought a second. A hiker who also mountain bikes calls it the perfect weight for days when the weather keeps changing, and another wears hers from September to May. CleverHiker found it a little itchy for the first few wears, and its light women's colors a bit see-through.

Flaws but not dealbreakers

  • Not warm enough on its own for a winter day standing still.
  • Thin fabric wears faster than a midweight.
  • A little scratchy for some people at first.

Fit and sizing

Slim fit, close to the body.

3
Merino.tech Midweight 250 Base Layer Set
Top and bottoms

Merino.tech Midweight 250 Base Layer Set

A merino top and long johns in the same weight as the Smartwool, for about what Smartwool charges for the top alone.

$89.99$129.9931% off
See it on Amazon
Top and long johns17.5 micron woolRated 0 to 30°F on the moveWomen's set too
+Read the full write-up

Why I picked it

Merino.tech sells a midweight merino top and long johns together for about $100, with a pair of wool socks thrown in. The fabric is the same 250 grams per square meter as the Classic Thermal, in fine 17.5-micron wool, which Merino.tech says is too fine to itch. Merino.tech rates it from 30°F down to 0°F while you're moving. The Merino.tech crew is OutdoorGearLab's best value in merino.

If you need thermals for a cold camping trip, a deer stand or a ski week, this is the cheapest way to get real wool on both halves.

Who it is for

Anyone who wants long underwear, top and bottoms, on a budget, and doesn't need a slim technical cut.

What owners report

Owners call it soft and warm, and wear it on ski trips and around the house all winter. The bad review is a hole: a mother found a large one in the middle of her son's top after his first day of skiing, while the sets she bought for herself and her youngest had none so far. Another owner points out that moths love wool, and puts his sets back in their plastic bag after washing.

Flaws but not dealbreakers

  • One owner found a hole after the first day, so look each piece over when it arrives, while you can still return it.
  • Wash it cold and dry it flat: Merino.tech warns that hot water and the dryer shrink merino.
4
Minus33 Expedition Weight Crew
Extreme cold

Minus33 Expedition Weight Crew

Thick 400-gram merino for ice fishing, hunting stands and long stops in deep cold. About $150, and you can put it in the dryer.

$149.99
See it on Amazon
17 oz in a largeMachine wash and dryTall sizes2-year warranty
+Read the full write-up

Why I picked it

For real cold, when you're standing still more than you're walking. The fabric is 400 grams per square meter of merino, the top weighs about 17 ounces in a large, and it's closer to a thin wool sweater than to a T-shirt. Unlike most wool, it goes in the washer and the dryer.

The fit has a little room in it, so it also layers as a midlayer over a thin top. It comes from XS to 4XL with tall sizes, and Minus33 covers it for two years.

Who it is for

Ice fishers, hunters, winter campers and anyone who stops for long stretches below 20°F. For hiking, it's too warm once you get moving.

What owners report

Owners talk about the warmth first: one bought it to keep his core warm while ice fishing, and it did. They also call it soft, and the ones who mention size say to order your usual one.

Flaws but not dealbreakers

  • Far too warm for most hiking.
  • Heavy and bulky in a pack.

Fit and sizing

Regular fit, true to size, with tall sizes from M to 3XL.

5
Under Armour ColdGear Armour Fitted Crew
Synthetic under $50

Under Armour ColdGear Armour Fitted Crew

A brushed, stretchy synthetic that dries fast after a sweaty climb, with mesh panels in the back. About $40.

$50.22$60.0016% off
See it on Amazon
Brushed interiorMesh back and underarmsSnug, not compressionTall sizes
+Read the full write-up

Why I picked it

Under Armour's best-known base layer. The fabric is two layers: brushed and soft against your skin, smooth and fast-drying outside, with stretch mesh in the back and under the arms where you sweat most under a pack. It costs about $40 on Amazon, against about $60 at Under Armour.

Under Armour says its ColdGear line works from about 0 to 55°F. It's a good layer for cold hikes and winter runs, and it goes in the washer and the dryer.

Who it is for

Hikers who sweat a lot, want something cheap that dries fast, and don't mind washing it after every outing.

What owners report

A runner says it kept him warm at 35°F, and owners say it pulls the sweat away and dries quickly after a workout. It holds up in the wash. A man at 6'3" with a 46-inch chest found the XL just right, with sleeves almost to his wrist. Complaints are rare; one parent's son simply doesn't like the high neck.

Flaws but not dealbreakers

  • Synthetic holds odor sooner than wool. On a multi-day trip, take merino.
  • The snug fit isn't for everyone.

Fit and sizing

Snug, close to the skin without the squeeze of compression. Your usual size, and tall lengths exist.

6
32 Degrees Heat Lightweight Crew
Under $15

32 Degrees Heat Lightweight Crew

A thin, stretchy thermal for about $10. Not the warmest, but warm enough when you're moving, and cheap enough to buy two.

$9.99
See it on Amazon
About 3.5 ozFittedAnti-odor finishWomen's top and leggings
+Read the full write-up

Why I picked it

For about $10, this is the thermal you buy to see whether base layers suit you, or to leave in the car for the day the forecast is wrong. It's thin, stretchy and fitted, about 3.5 ounces, and goes under a shirt without bulk. It isn't as warm as wool, but it's warm enough while you're walking.

It's Outdoor Life's budget pick, and their reviewer wore the same top for seven years and found no holes, just light pilling, although the bottoms got runs and holes.

Who it is for

Beginners, kids who outgrow everything, and anyone who wants a spare.

Flaws but not dealbreakers

  • Not warm enough on its own once you stop.
  • It pills a little with time.

Fit and sizing

Fitted, in your usual size.

Who this is for

This guide covers the layer next to your skin when you hike or camp in the cold, under a fleece or an insulated jacket, and the long underwear you sleep in on a cold night. If you're packing for a weekend at the campground, our camping checklist has the rest of the clothing. If you sleep cold, a dry base layer and a warmer sleeping bag do more together than either one alone.

Each pick links to the men's version, and each one comes in a women's cut too, from Smartwool, Merino.tech, Minus33, Under Armour and 32 Degrees.

How to choose

Merino wool or synthetic

Merino stays warm when it's damp, breathes on the climb and doesn't stink after a few days, so it's the best base layer for most hikers. It costs more and wears faster. Synthetic is cheaper, tougher and dries faster, but it smells after a single sweaty day. Wool for trips, synthetic for workouts and tight budgets.

Pick the weight for the temperature

Lightweight (about 150 grams per square meter) for hiking in cool weather, or for anyone who runs warm. Midweight (about 250) for cold days and most winter hiking. Heavyweight (400 and up) for extreme cold and standing still. If you only buy one, buy a midweight.

For hiking, go one weight lighter

You warm up fast on a climb, and a layer that's soaked with sweat at the top chills you as soon as you stop. Wear the lighter top on the move and keep a warm jacket in the pack for breaks. The heavier layers are for camp and the cold hours.

Close to the skin, not tight

A base layer only moves sweat if it touches you. It should hug your body without pinching at the waist, the neck or the wrists. Check the sleeve length too: cuffs that ride up leave your wrists cold.

The competition

Icebreaker Merino 200 Oasis Crewe

OutdoorGearLab's favorite for women, about $120. It's thinner than the Classic Thermal at the same price, so I'd take the Smartwool for winter.

Smartwool Intraknit Thermal Merino Crew

A merino blend with mesh zones knit in, made for hard efforts in the cold. CleverHiker found it very warm for its weight, but it costs about $130 and CleverHiker found it less durable.

Carhartt Force Midweight Base Layer

A synthetic and merino blend for about $65, built for work sites more than trails. The ColdGear costs less and the Merino.tech set gives you real wool for not much more.

Meriwool Merino 250 Half Zip

Another budget merino at about $70, with a zip at the neck to let heat out. A fair alternative to Merino.tech, with less written about it.

Helly Hansen Lifa Merino Midweight Crew

Helly Hansen's merino midweight, about $105. A good layer, but for that price I'd take the all-wool Classic Thermal.

Patagonia Capilene Midweight

One of the best synthetics, picked by CleverHiker and Treeline Review. Patagonia doesn't sell it on Amazon, so it isn't on this list.

How we picked

Sold as a base layerPublished fabric weight or weight classMen's and women's versionsOn Amazon under $150Named in a major gear guide or well reviewed by owners

I looked at 27 base layers named by the leading outdoor guides. Fabric weights come from the makers or from Treeline Review, garment weights from the makers, Treeline, CleverHiker and Outdoor Life.

Owner stories come from Zappos and from the reviews on Merino.tech's and Minus33's own sites, and the long-term wear notes from Treeline Review, CleverHiker and Outdoor Life.

Common questions

Do you wear underwear under a base layer?+

Yes, most people do, and it's normal to wear your usual underwear under long underwear. Pick wool or synthetic underwear, not cotton, so the layer underneath doesn't hold sweat against you.

Is Under Armour good for base layers?+

Yes, for a synthetic. The ColdGear Armour Fitted Crew is warm, fast-drying and about $40. It smells sooner than merino, so it's better for day hikes and workouts than for a week on the trail.

What are the best base layers for women?+

The same ones, in women's cuts. The Smartwool Classic Thermal for cold days, the All-Season for hiking, and the Merino.tech women's set for top and bottoms on a budget.

How tight should a base layer be?+

Close to your skin everywhere, so it can move sweat away, but loose enough that it doesn't pinch at the waist, the wrists or under the arms.

Is merino wool itchy?+

Not usually. Base layer merino is much finer than the wool in an old sweater, and most people feel nothing. Some find a new top a little scratchy for the first few wears. If wool always bothers you, the Under Armour or the 32 Degrees are the synthetic choice.

The verdict

Start with the Classic Thermal

For about $125, the Classic Thermal gives you all-merino warmth for winter hikes, a fit that sits close under a pack, and bottoms to match.

If you'd rather have top and bottoms for about $100, the Merino.tech set is the set. Spring and fall hikers do better in the thinner All-Season, and the Minus33 Expedition is for standing still in deep cold.

Check price on Amazon$125.00

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