Mens Classic Thermal Merino Base Layer Bottom-Base Layers-Smartwool-Black-Mountain Sports
Smartwool

Men's Classic Thermal Merino Base Layer Bottom

Smartwool's warmest everyday Merino base layer, with a functional fly and a Merino-lined waistband.

$125
Color: Black
Also inWomen'sUnisex
Size:
Size guide
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Top features
100% ZQ Merino wool
Moisture management
Midweight 250 g/m² interlock knit
Description

We've updated our Men's Classic Thermal Merino Base Layer Bottom to bring you a better fitting, more functional tight. The technical benefits of 100% Merino wool will help you keep your game at a high level for longer. And our heaviest weight wool fabric will help keep you warm and comfortable as you ski, run, and hike the day away.

Perfect for
Ski and ride
Under a shell pant.
Run and hike
Cold-weather days.
ZQ Merino
Certified wool.
Features
  • 100% ZQ Merino wool. Smartwool sources this fabric through the New Zealand Merino Company's ZQ programme, which covers animal welfare, environmental and social standards.
  • Moisture management. Merino moves moisture as vapor before it becomes sweat on the skin, so you stay drier under a shell on start-stop days.
  • Midweight 250 g/m² interlock knit. The Classic Thermal weight — built for cool to cold weather rather than high-output summer use.
  • Fully functional fly. A real fly, not a mock one — which matters on a base layer worn under a bib or a shell pant all day.
  • Flatlock seams and Merino-lined waistband. Seams lie flat to minimize chafing, and the elastic waistband is lined with Merino so no synthetic sits against the skin.
Size & Fit
Slim
Regular
Relaxed

Close to the body. Size up if you're between sizes or want room to layer underneath.

Slim fit, intended to follow the contour of the body. Take your usual size for a close base-layer fit; size up if you prefer more room or plan to layer underneath. Merino-lined elastic waistband, so no synthetic elastic sits against the skin. Inseam is 30 inches (76 cm) on a size medium and scales across the size run.

Open size guide
Materials
  • 100% Merino wool.
  • Care: Machine wash cold on gentle cycle with like colors; do not bleach; tumble dry low or lay flat to dry; do not iron; do not dry clean.
About the brand

Smartwool brings the magic of merino wool to life — keeping you warm, dry, and comfortable no matter the season. From performance socks to base layers, every piece is designed for adventure. At Mountain Sports, we love Smartwool because it proves comfort and sustainability can go hand in hand, from trail to town.

Shop all Smartwool

SKU SW016361-001 · Style SW016361
Size guide

Smartwool Size Chart - Men's Apparel

Smartwool Sizing Chart

Measurements are in inches, unless otherwise noted. Charts reflect body measurements, not garment measurements.

Men's Apparel & Base Layers:

Size Chest Waist Hip Arm Length Inseam
S 36 - 38 29 - 31 36 - 38 31 - 32 30 - 31
M 39 - 41 32 - 34 39 - 41 32 - 33 31 - 32
L 42 - 44 35 - 37 42 - 44 33 - 34 32 - 33
XL 45 - 48 38 - 40 45 - 48 34 - 35 33 - 34
XXL 49 - 51 41 - 44 49 - 51 35 - 36 34 - 35

How to measure - Chest: measure under the arms, around the fullest part of your chest and back. Waist: measure around your natural waistline, keeping the tape a bit loose. Hip: measure around the fullest part of your hip, parallel to the floor. Arm length: with your elbow slightly bent, measure from the center back of your neck, across the top of your shoulder, to the elbow and down to the wrist bone. Inseam: measure along the inside of the leg from crotch to ankle bone, or measure a well-fitting base layer pant from the crotch seam to the bottom of the leg.

Smartwool clothing runs true to size. Smartwool products are designed to retain their fit and shape; following the care instructions on the label will help avoid shrinking or stretching.
Performance

How it ranks across conditions.

Calibrated against comparable products we stock, so you can compare like for like.

Warmth

Light
Midweight
Warm
Very warm

At 250 g/m² this is Smartwool's Classic Thermal weight — the warmest of their everyday Merino base layer fabrics, built for cool to cold conditions.

Breathability

Low
Moderate
High
Very high

Merino breathes well for its warmth, but a 250-weight knit is denser than a 150 — expect to run warm on high-output days above freezing.

Odor resistance

Low
Moderate
High
Excellent

Merino resists odor build-up naturally, which is what makes a wool base layer wearable for several days on a hut trip.

Fit

Compression
Slim
Standard
Relaxed

Smartwool's slim fit follows the contour of the body so it layers cleanly under a ski or hiking pant without bunching.

Best for

Is this right for you?

Ski and snowboard days

Smartwool labels this weight start-stop activity, which is exactly what a lift day is. The functional fly earns its keep under a bib.

Not for: For sustained high-output cold-weather effort like skinning laps, Smartwool points to their Intraknit line instead. Try the lighter Merino 150 Base Layer Bottom

Cold-weather running and hiking

Flatlock seams sit flat under a harness or pack belt, and the slim leg layers under a softshell without bunching at the knee.

Not for: Above about 50°F this is more wool than most people want against the skin on the move. Try the lighter Merino 150 Base Layer Bottom

Everyday winter layering

Wears comfortably under work trousers on genuinely cold days, and resists odor well enough to go several wears between washes.

Not for: It is a base layer, not a legging — the slim cut is meant to sit under something, not be worn on its own. Pair it with the Classic Thermal Crew

Full spec sheet

Technical Specs

Material100% Merino wool, exclusive of decoration
Fabric weightMidweight — 250 g/m²
Garment weight228 g (8.04 oz)
FitSlim — follows the contour of the body
FlyFully functional
WaistbandMerino-lined elastic
Inseam (size M)30 in (76 cm)
Country of originMade in Vietnam
Style numberSW016361
Questions, answered

Before you buy.

Pulled from chats, emails and store visits. The things people actually ask us before they buy. Don't see yours?

Ask a question
How does the 250 weight compare to Smartwool's 150?

The 150 (Classic All-season) is a lightweight 150 g/m² fabric Smartwool positions for warm to cool weather — worn alone on warm days, as a layering foundation on cooler ones. This Classic Thermal is 250 g/m² and is made for cool to cold weather. If most of your days are above freezing and high-output, the 150 is the better pick.

Is the fly functional?

Yes. Smartwool builds this with a fully functional fly rather than a mock one, which is the difference between a base layer you can live in under a bib all day and one you cannot.

Do these run true to size?

Smartwool builds this in a slim fit intended to follow the contour of the body, sized to their standard men's chart. Take your usual size for a close base-layer fit; size up if you prefer more room or plan to wear something underneath.

Will 100% Merino hold up to regular washing?

Yes, with reasonable care. Machine wash warm on a gentle cycle and tumble dry low or lay flat. Skip fabric softener — it coats the fiber and blunts the wool's moisture management. Merino also needs washing far less often than synthetic, so it sees the machine less to begin with.

What does ZQ certified wool mean?

ZQ is a wool certification developed by the New Zealand Merino Company, covering animal welfare, environmental management and social standards on the farms that supply the fiber. Smartwool states that practices which cause harm, such as mulesing, are never permitted under it.

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