Women's Classic Thermal Merino Base Layer Bottom
Smartwool

Women's Classic Thermal Merino Base Layer Bottom

Smartwool's warmest everyday Merino base layer, cut slim to layer under a ski or hiking pant.

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

100% Merino. 100% capable. Our Women's Merino Base Layer Bottom is always ready for what's ahead and what's next. With a fit updated to improve functionality while enhancing aesthetics for a range of body shapes and a flatlock seam construction designed to minimize chafing, comfort is never in question. Get yourself a pair or two of our best base layer pants and conquer the cold with more confidence than ever.

Perfect for
Ski and ride
Under a shell pant.
Cold-weather hiking
Start-stop 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.
  • Flatlock seam construction. Seams lie flat against the skin to minimize chafing under a pack strap, harness or ski pant.
  • Merino-lined waistband. The elastic waistband is lined with Merino, so no synthetic elastic sits directly against the skin at the waist.
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 contours 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. Mid-rise waistband, lined with Merino so no synthetic elastic sits against the skin. Inseam is 30.5 inches (77.5 cm) on a size medium and scales across the size run. Offered XS to XL.

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 SW019242-001 · Style SW019242
Size guide

Smartwool Size Chart - Women's Apparel

Smartwool Sizing Chart

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

Women's Apparel & Base Layers:

Size Numeric Size Equivalent Chest Waist High Hip Hip Inseam Arm Length
XS 0 - 2 32 - 33 24 - 25 31 - 32 35 - 36 29 - 30 27 - 28
S 4 - 6 34 - 35 26 - 27 33 - 34 37 - 38 30 - 31 28 - 29
M 8 - 10 36 - 38 28 - 30 35 - 36 39 - 40 31 - 32 29 - 30
L 12 - 14 39 - 40 31 - 34 37 - 39 41 - 43 32 - 33 30 - 31
XL 16 - 18 41 - 44 35 - 38 40 - 43 44 - 47 33 - 34 31 - 32

Women's Plus Sizes:

Size Numeric Size Equivalent Chest Waist High Hip Hip Inseam Arm Length
1X 16 - 18 43 - 45 39 - 41 44 - 46 45 - 47 30 - 32 31
2X 20 - 22 47 - 49 43 - 46 48 - 50 49 - 51 30 - 32 31
3X 24 - 26 51 - 53 47 - 50 52 - 54 53 - 55 30 - 32 32
4X 28 - 30 55 - 57 52 - 55 56 - 58 57 - 59 30 - 32 33

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 comfortably loose. High hip: measure around your high hip, approximately 3 inches below your natural waistline. Hip: measure around the fullest part of your hip, parallel to the floor. 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. 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.

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 — their warmest everyday Merino base layer fabric, positioned 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 contours 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's own label for this weight is start-stop activity — exactly what a lift day is. Warm on the chair, breathable enough on the way down.

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 hiking

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

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 jeans or 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 to 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 weight208 g (7.34 oz)
FitSlim — follows the contours of the body
RiseMid-rise, Merino-lined elastic waistband
Inseam (size M)30.5 in (77.5 cm)
Country of originMade in Vietnam
Style numberSW019242
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.

Do these run true to size?

Smartwool builds this in a slim fit intended to follow the contours of the body, and sizes it to their standard women's chart. Take your usual size for a close base-layer fit. If you prefer a little more room, or plan to wear another layer underneath, size up.

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.

How long is the inseam?

Smartwool publishes a 30.5 inch (77.5 cm) inseam on a size medium, and it scales with the size run. The leg is cut to sit under a ski or hiking pant without bunching at the ankle.

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