Thermal Underwear Size Chart (Base Layer Fit)
Thermal underwear (long johns and base-layer tops) works by trapping a thin layer of warm air against the skin, so unlike most clothing it's meant to fit snug, not loose. Size to your body, not with room to spare.
Thermal Underwear Size Chart (to-fit body, inches)
| Size | Chest | Waist |
|---|---|---|
| XS | 32–34 | 26–28 |
| S | 35–37 | 29–31 |
| M | 38–40 | 32–34 |
| L | 41–43 | 35–37 |
| XL | 44–46 | 38–40 |
| XXL | 47–49 | 41–43 |
Notes
Snug is the point. A base layer should sit close to the skin so it can trap warmth and wick moisture. A loose thermal leaves cold air gaps and doesn't wick well — buy true to size, and if between sizes, size down for a proper base-layer fit.
It has to fit under your clothes. Thermals go beneath your normal layers, so bulk works against you. A close fit slides under jeans and shirts without bunching; an oversized one bunches and shows through.
Check the fabric stretch. Merino and synthetic base layers usually have four-way stretch, so they hug without restricting. If a thermal is a non-stretch waffle knit, don't size down — it won't give the way a stretch base layer does.
FAQs
Should thermal underwear be tight?
Yes — a base layer should fit snug against the skin to trap warm air and wick sweat. Loose thermals leave cold gaps and don't perform. Buy true to size, and size down rather than up if you're between sizes.
How do I measure for thermals?
Measure your chest and waist at the fullest points and match the chart directly, without adding room. Because base layers are meant to be close-fitting (and usually stretch), you size to your actual body measurements.
Can I wear thermals under regular clothes?
Yes — that's their purpose. A snug base layer slides under jeans, trousers, and shirts without bunching. Choose a stretch merino or synthetic for the least bulk and best moisture-wicking under your normal clothing.
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