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

Kids' Classic Thermal Merino Base Layer Bottom

Smartwool's warmest everyday Merino for kids, in a slim bottom that layers under snow pants without bunching.

$70
Color: Black
Also inWomen'sMen's
Size:
Size guide
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Top features
100% ZQ Merino wool
Midweight 250 g/m² interlock knit
Moisture management
Description

Our Kids' Classic Thermal Merino Base Layer Bottom uses our super-soft interlocking knit, perfect for layering in cool conditions and great for wearing alone on a mild day. Now your kids can be as warm, dry, and comfortable as you are.

Perfect for
Ski and ride
Under snow pants.
Everyday warmth
Cold mornings.
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.
  • Midweight 250 g/m² interlock knit. The Classic Thermal weight — built for cool to cold weather rather than high-output summer use.
  • Moisture management. Merino moves moisture as vapor before it becomes sweat on the skin, so kids stay drier through a day of stop-start play.
  • Merino-lined elastic waistband. The waistband is lined so 100% Merino stays next to skin, with no elastic edge rubbing at the waist.
  • Flatlock seam construction. Flat seams sit against the skin without a ridge, which is what keeps a base layer from chafing under snow pants.
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, so it layers cleanly under snow pants. Take your child's usual size; the Merino-lined elastic waistband gives without digging in. Offered XXS to XL — see the Smartwool kids' chart for the age and height each size maps to.

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 SW0SN112-001 · Style SW0SN112
Size guide

Smartwool Size Chart - Kids' Apparel

Smartwool Sizing Chart

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

Kids' Apparel & Base Layers:

Size Age Chest Waist Height Weight (lbs)
XXS 2 - 3 22.5 - 23.5 20 - 21.5 35 - 41 31 - 41
XS 4 - 5 24 - 25 22 - 22.5 42 - 48 42 - 49
S 6 - 8 25.5 - 26.5 23 - 24 48 - 54 49 - 59
M 9 - 10 27 - 28 24.5 - 25 54 - 57 59 - 75
L 11 - 12 28.5 - 30 25.5 - 26 57 - 60 75 - 89
XL 13 - 14 30.5 - 32 26.5 - 28 60 - 62 89 - 100

How to measure - Chest: measure under the arms, around the fullest part of the chest and back. Waist: measure around the natural waistline, keeping the tape a bit loose. Height: measure standing straight, without shoes, from the top of the head to the floor.

Smartwool clothing runs true to size. If your child is between sizes, age, height and weight are good guides for choosing the best fit. 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 a warm kid on mild days above freezing.

Odor resistance

Low
Moderate
High
Excellent

Merino resists odor build-up naturally, which is why a wool base layer survives a week of ski school between washes.

Fit

Compression
Slim
Standard
Relaxed

Smartwool's slim fit follows the contours of the body so it layers under snow pants without bunching at the knee.

Best for

Is this right for you?

Ski and snowboard days

Warm enough for the chair and breathable enough for the run down — this weight is built for the stop-start rhythm of a lift day.

Not for: Not the pick for a warm spring day; above freezing most kids run hot in a 250-weight bottom. Pair it with the Classic Thermal Crew

Cold-weather everyday layering

Slim enough to disappear under school trousers or jeans on a genuinely cold morning.

Not for: It is a base layer, not a standalone pant — it is cut close and not meant to be worn on its own outdoors. Add the Kids' Thermal Merino Neck Gaiter

Sledding and winter play

Merino keeps working when it gets damp, which is what happens to every kid who spends an afternoon in the snow.

Not for: For high-output cold-weather effort, Smartwool points to their Intraknit line rather than Classic Thermal. Add the Kids' Merino Gloves

Full spec sheet

Technical Specs

Material100% Merino wool, exclusive of decoration
Fabric weightMidweight — 250 g/m²
Garment weight125 g (4.41 oz)
FitSlim — follows the contours of the body
ConstructionMerino-lined elastic waistband, flatlock seams
CertificationZQ Merino — animal welfare, environmental and social standards
Country of originMade in Vietnam
Style numberSW0SN112
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
What size should I order for my child?

Take their usual size. The fit is slim and meant to follow the contours of the body, so sizing up mostly adds bunching under snow pants rather than room to grow. The size range is XXS to XL — the Smartwool kids' chart on this page maps each to age and height.

Will it be itchy?

Merino is a much finer fiber than the wool most people remember from childhood, and this is an interlock knit designed to sit against the skin. Most kids who dislike wool sweaters are fine in Merino base layers. If your child is genuinely wool-sensitive, try it on before a full ski day.

How often does it need washing?

Far less often than a synthetic base layer. Merino resists odor build-up, so airing it out overnight is usually enough between wears. Wash it when it is actually dirty.

Will it shrink?

Not if you follow the label. Machine wash cold on a gentle cycle and tumble dry low or lay flat. Heat is what shrinks and felts wool — a hot dryer is the one thing that will ruin it.

What does the 250 in Classic Thermal mean?

It is the fabric weight in grams per square meter. 250 g/m² is Smartwool's midweight — warmer than their 150 Classic All-season, and the weight they position for cool to cold weather.

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