Merino long johns are thin, warm thermal layers worn under regular pants. A midweight pair (190–250 gsm) adds real warmth without visible bulk under jeans, work trousers, or ski pants — and stays fresh for several days of wear thanks to merino's natural odour resistance.
What are merino long johns?
Long johns are thermal leggings worn under regular pants for warmth. Merino long johns are made from merino wool rather than cotton or synthetic fabric — which means they regulate temperature better, don't smell, and stay warm when damp.
For women, the same garment is often called merino leggings or merino base-layer leggings. The construction is similar; the cut is different.
Do I actually need long johns?
If you spend significant time outside in cold weather (winter walks, ski lift queues, fishing, outdoor work, cold commutes), long johns make a meaningful difference. The legs lose heat fast, especially in wind, and a thin layer of merino under your usual trousers does most of the work of a thick pair of pants without the bulk.
Common situations where long johns earn their keep:
- Skiing and snowboarding (under ski pants).
- Winter cycling and walking commutes.
- Outdoor work in cold weather (farming, construction, frontline roles).
- Fishing — especially boat fishing and pre-dawn starts.
- Hiking and tramping in winter or alpine conditions.
- Cold offices and unheated workplaces.
- Sleeping in cold huts or tents.
What weight long johns?
- Lightweight (130–190 gsm): mild-cold conditions, high-output activities, layering under ski pants on warmer days, summer alpine sleeping layer.
- Midweight (190–250 gsm): the all-rounder. Skiing, winter commute, everyday cold-weather wear, hunting, fishing.
- Heavyweight (250+ gsm): very cold conditions, low-activity outdoor work, winter sleeping, people who feel the cold.
How should merino long johns fit?
Snug, but not tight. The fabric should make contact with skin all the way down to wick moisture and trap warmth, but it shouldn't restrict movement or pinch at the waist or knees. A correctly fitted pair feels almost like a second skin once you're wearing them.
Most modern merino long johns include:
- A wide elastic waistband that doesn't dig in.
- A gusseted crotch for free movement.
- An ankle cuff that stays in place inside socks or boots.
- A flat-stitched seam or no front fly (a fly is rare on merino because the fabric stretches enough not to need one).
Will long johns show under my jeans?
Lightweight and midweight merino long johns are thin enough that they don't show under standard-cut jeans, work trousers, or hiking pants. Heavyweight long johns can show as visible bulk under fitted trousers — keep those for outerwear-over situations rather than office or fitted-trouser wear.
Long johns vs leggings vs thermal underwear — what's the difference?
- Long johns: traditional men's term. Legged thermal underwear, usually with a waistband and ankle cuffs.
- Leggings: usually the women's-cut equivalent. Same garment, different fit through hips and waist.
- Thermal underwear: umbrella term covering both, plus tops.
- Base-layer bottoms: the technical-fabric industry name.
They're all variations of the same thing.
Frequently asked questions
Are merino long johns warmer than synthetic long johns?
For the same weight, slightly. The bigger advantages are that merino stays warm when damp from sweat (synthetics get cold), regulates temperature better in changing conditions, and doesn't smell after multiple days of wear.
How many days can I wear merino long johns between washes?
Three to five for most people. Merino's antibacterial properties resist odour, so unless they're visibly dirty or you've been doing something genuinely sweaty, they don't need daily washing.
Can I wear merino long johns to bed?
Yes — they make excellent cold-weather sleepwear. Many people find them more comfortable than pyjama bottoms because they don't twist or ride up overnight. They're also the standard sleeping layer for winter camping and cold huts.
Do merino long johns stretch out over the day?
Quality merino long johns hold their shape through a full day of wear. If they're stretching out badly, they're probably either too large or made from a weak merino blend rather than 100% merino.
How do I wash merino long johns?
Cool gentle cycle, wool detergent, dry flat. Don't tumble dry — heat and agitation will felt the fibres and shrink the garment. See our full merino care guide.
Are there long johns that work for both men and women?
Most brands offer separate cuts for men's long johns and women's leggings because the garments fit differently through the hips, waist, and rise. A women's leggings cut on a man's body (or vice versa) usually fits poorly. Buy the cut that matches your body shape.
What's the difference between long johns and ski thermal pants?
Often nothing — they're the same garment marketed for different uses. A pair of midweight merino long johns is what most ski-shop "ski thermals" actually are. Buying them as long johns rather than as ski-specific thermals usually costs less.
Brass Monkeys merino long johns
Smart Merino's Brass Monkeys merino long johns are made in New Zealand from 100% merino wool. Designed with a wide elastic waistband, dual-layered crotch gusset, and extra leg length so they stay in place through a full day of wear. Available in lightweight, midweight, and heavyweight. The matching women's merino leggings use the same fabric and construction in a women's cut.
