Merino Wool Weights Explained

Not all merino wool is the same fabric. Interlock, wool fleece, boiled wool, and terry are the four main types — each made differently, each suited to different clothing. Picking the right weight is the difference between a wool base layer that feels like a second skin and a boiled wool jacket that holds up to a winter storm. Here's how each type is made, how they compare, and which weight to choose for what.

Two systems, one fabric

European terminology vs. GSM

There are two ways wool fabric gets described, and they talk about the same thing from different angles.

European heritage brands — Disana, Engel, Reiff, Hirsch-Natur — describe wool by construction type. You'll see terms like "interlock," "wool fleece," "boiled wool," and "terry." These describe how the fabric is knitted or finished.

Performance brands popular in North America — Icebreaker, Smartwool, Minus33 — describe wool by GSM (grams per square meter), which is pure fabric weight. You'll see numbers like 150 GSM, 200 GSM, 250 GSM.

Neither system is better. European terminology is more descriptive of what the fabric actually feels and behaves like. GSM is more precise if you want to compare two garments on pure warmth. This guide uses European terms first — that's what our catalog uses — and includes GSM equivalents for reference.

Quick comparison

The four main wool fabric types at a glance

	Interlock	Wool fleece	Boiled wool	Terry
GSM range	140–220	250–350	400–500+	200–300
Feel	Smooth, stretchy, thin	Fluffy, plush, soft	Dense, felted, slightly stiff	Looped, absorbent
Best for	Base layers, T-shirts, leggings	Baby booties, thermal pants, soft outerwear	Coats, jackets, snowsuits	Diaper inserts, some towels
Wear position	Next-to-skin	Mid-layer or outer	Outer layer only	Varies by use
Warmth	Light to moderate	Warm	Very warm, wind-resistant	Moderate

Type #1

Interlock: the base-layer standard

Interlock is a smooth, stretchy knit that feels closer to a soft cotton T-shirt than to traditional wool. It's made by knitting two layers of jersey together, which creates a stable, even fabric that doesn't curl at the edges and stretches in all directions.

This is the fabric you want for anything worn directly against the skin: base layer tops, long underwear, leggings, T-shirts, bodysuits, and sleepwear. It's thin enough to layer under other clothing without bulk, but warm enough to wear alone in mild cold.

Lightweight (140–180 GSM)

The thinnest interlock. Perfect for warm-weather base layers, summer wool, and next-to-skin items. Engel's lightweight base layers fall here.

Midweight (190–220 GSM)

Slightly warmer and more substantial. Works as a standalone top in cool weather or as a winter base layer. The typical weight for Engel and Reiff's adult thermal tops.

Wool-silk interlock

Engel's 70/30 wool-silk blend is still technically interlock but woven from blended yarn. Softer drape, slightly cooler than pure wool, excellent for sensitive skin.

When to choose interlock

Anything worn next to the skin. Base layers for skiing or hiking, everyday undershirts, baby bodysuits, pajamas, and leggings all fall here. If a garment needs to stretch and move with the body, interlock is almost always the answer.

Type #2

Wool fleece: plush and warm

Wool fleece is knitted wool that's been brushed on one side, raising the fibers into a soft, fluffy pile. The result looks and feels similar to synthetic fleece — but it's 100% wool, with all of wool's natural thermal and moisture-wicking benefits.

It's warmer than interlock at the same weight because the brushed fibers trap air, and softer than boiled wool because the fabric stays open and flexible. For baby clothing especially, wool fleece is the gold standard — it's what makes Engel's baby booties so plush and Reiff's baby rompers so warm without being stiff.

Typical weight

250–350 GSM. Heavier than interlock, lighter than boiled wool. Most brands don't publish GSM on fleece items — the "fleece" designation is the weight indicator.

Best uses

Baby pants, thermal leggings, booties, mittens, cardigans, and soft outerwear. Also used as a mid-layer worn over an interlock base layer.

Against the skin?

For most people and most babies, yes. The brushed surface is soft enough for direct wear. For very sensitive skin, layer it over a thin interlock or cotton base.

When to choose wool fleece

When you want warmth without stiffness. Ideal for baby and toddler garments, cold-weather thermal layers, and anything where plushness matters. Wool fleece is also more forgiving than boiled wool — it drapes well, fits easily, and doesn't require the same precise sizing.

Type #3

Boiled wool: structured outerwear

Boiled wool is a knitted wool that's been intentionally shrunk through hot water and agitation — essentially, controlled felting. The result is a dense, slightly stiff fabric that's wind-resistant, exceptionally warm, and holds its shape for years.

Disana is the heritage brand most associated with boiled wool. Their baby snowsuits, kids' jackets, and adult coats are made from this fabric, and they've been refining the process since the 1970s. Boiled wool is the traditional fabric of European winter outerwear for good reason: it works.

Typical weight

400–500 GSM or more. The heaviest wool fabric in regular use. Dense, firm, and noticeably structured compared to other wool.

Best uses

Coats, jackets, snowsuits, outer layers for serious cold. Also used for skirts, slippers, and structured pieces where shape matters.

Against the skin?

No — boiled wool is designed to be worn over other clothing. It's too dense and slightly stiff for direct skin contact.

When to choose boiled wool

Real winter outerwear where warmth and wind resistance matter. Baby snowsuits, kids' winter coats, and adult jackets all benefit from boiled wool's density. It's also exceptionally durable — a well-made boiled wool jacket will outlast two or three synthetic replacements.

Type #4

Terry: the specialist fabric

Wool terry is a knitted wool with looped pile on one or both sides — the same construction as cotton terry towels, but in wool. It's less common than the other three types and serves more specialized uses.

In the baby clothing space, terry shows up mostly in sleepers and certain bodysuits where the absorbent looped structure matters. In cloth diapering, wool terry is sometimes used for diaper inserts because of its absorbency and moisture management. Outside those specific uses, you'll rarely see it in everyday garments.

Typical weight

200–300 GSM, depending on pile density. Not as heavy as fleece or boiled wool.

Best uses

Baby sleepers, diaper inserts, certain towels. Any application where the looped pile's absorbency is useful.

Feel

Soft with visible loops on one side. Can feel slightly textured against the skin. Not as smooth as interlock, not as plush as fleece.

The blend factor

Wool-silk: the premium next-to-skin choice

Wool-silk blends — typically 70% merino wool and 30% silk — are their own category, distinct from the four main wool fabric types. The blend is almost always interlock construction, but the silk transforms how the fabric feels and performs.

Silk isn't just a softening agent. It's a thermoregulating fiber in its own right — lightweight, breathable, and naturally temperature-balancing, which is why silk has been used as a base layer fabric for centuries. Combined with merino wool, it creates a fabric that's warm in cold weather, cooler in warm weather, exceptionally soft against the skin, and genuinely luxurious to wear.

The silk adds a subtle sheen, a smoother drape, and a hand feel closer to cashmere than to typical merino. For everyday wear, base layers, sleepwear, and infant bodysuits, wool-silk is the premium choice — the fabric you reach for when quality matters. Engel's wool-silk pieces, including their lace-trimmed women's base layers and infant bodysuits, are prime examples of what this blend can do.

The only real tradeoff is price. Silk is more expensive to produce than wool, and wool-silk garments cost more than their 100% merino equivalents. For most buyers who try it, the comfort and quality justify the difference.

When to choose wool-silk

For any next-to-skin application where comfort and quality are priorities — newborn and infant wear, everyday base layers, sleepwear, and pieces for sensitive skin or eczema. It's also the right choice when you want wool's benefits in a fabric that feels and drapes more like fine apparel than technical gear.

How to choose

Picking the right wool weight for your use case

Shortcut: start with what you'll do in the garment, then work backward to the fabric.

You need

Best fabric

Base layer for skiing, hiking, or cold-weather activity

Midweight interlock (200 GSM)

Everyday T-shirt or undershirt in wool

Lightweight interlock (140–180 GSM)

Baby bodysuit or sleep romper

Lightweight interlock or wool-silk

Baby pants, booties, or warm layers

Wool fleece

Winter coat, jacket, or snowsuit

Boiled wool

Cardigan or sweater as a layer

Wool fleece or heavier interlock

Sensitive skin or eczema

Wool-silk interlock

Diaper cover (waterproof)

Double-knit interlock (lanolized)

FAQ

Frequently asked questions

What weight of merino wool should I buy for a base layer?

Lightweight interlock (around 140 to 180 GSM) is the standard choice for base layers. It's smooth, stretchy, and designed for next-to-skin wear. Midweight interlock (200 GSM) works for colder conditions or as a standalone layer. For outdoor activity in very cold weather, pair a lightweight interlock with a wool fleece or boiled wool outer layer rather than buying a single heavyweight base layer.

What's the difference between wool fleece and boiled wool?

Wool fleece is a knitted wool with a brushed, fluffy texture — plush, soft, and warm, typically used for baby garments and thermal layers. Boiled wool is a knitted wool that has been intentionally shrunk through heat and agitation, creating a dense, felted, slightly stiff fabric used for outerwear like jackets and coats. Fleece is softer and more drapey; boiled wool is firmer and more structured.

What is GSM and how does it compare to European wool terminology?

GSM (grams per square meter) is a measure of fabric weight used by brands like Icebreaker and Smartwool. European brands like Disana, Engel, and Reiff typically describe wool by construction type (interlock, fleece, boiled, terry) instead. Roughly: lightweight interlock ≈ 140-180 GSM, midweight interlock ≈ 200 GSM, wool fleece ≈ 250-350 GSM, boiled wool ≈ 400-500+ GSM. Both systems describe the same thing, just from different angles.

What wool weight is best for babies?

For next-to-skin baby wear — bodysuits, base layers, sleep rompers — choose lightweight interlock or wool-silk blends. Wool fleece is ideal for warmer outer layers like pants, booties, and cardigans where softness and warmth matter most. Boiled wool is reserved for coats, jackets, and snowsuits — outerwear that sits over other clothing rather than against the skin.

How does wool-silk compare to 100% wool?

Wool-silk blends — typically 70% merino and 30% silk — combine two thermoregulating natural fibers into a single premium fabric. The blend feels softer and drapes more smoothly than pure merino, with a subtle sheen and hand feel closer to cashmere. Silk's own thermal properties mean you don't sacrifice warmth for softness, and the blend is often preferred for next-to-skin wear, infant clothing, and anyone with sensitive skin or eczema. The tradeoff is price — wool-silk costs more than 100% merino.

What weight should I choose for outerwear?

For true outerwear — coats, jackets, snowsuits — boiled wool is the traditional choice. It's dense, wind-resistant, and holds its shape. For lighter outerwear layers like cardigans or sweaters worn over other clothes, wool fleece or heavier knitted interlock works well. Match weight to climate: boiled wool is built for sustained cold, while wool fleece suits milder winter conditions.

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