Thermal underwear should fit closer to the body than a regular T-shirt or sports pants. But the difference between a snug fit and a size that is simply too small is quickly apparent: just raise your arms, squat 3–5 times, or walk around for a few minutes. A set that is too loose will bunch up under fleece and shift as you move. A set that is too tight will pull at the shoulders, underarms, waist, or knees. You might not notice it during a quick try-on, but after 2 hours hiking, training, or working with gear, the fit becomes very noticeable.
That is why we recommend not relying solely on your usual “I always wear L.” For thermal underwear, it is better to spend a few minutes taking your measurements and checking them against the size chart for that specific model. If you have doubts about buying online, our manager will help you compare your measurements.
What measurements do you need before ordering?
You will need a regular measuring tape. It is best to measure over your body or over a thin T-shirt, without fleece, hoodies, or thick pants.
Before choosing a size, check:
- chest circumference — around the widest part of your rib cage;
- waist circumference — around the area where the thermal pants will actually sit;
- hip circumference — around the widest part;
- height — this helps assess the length of the sleeves, back, and pant legs.
Do not pull the tape tight. It should lie flat without sagging, but it should not dig into your body.
Next, compare the numbers with the chart for the exact model you plan to order. The labels M, L, or XL do not tell you much on their own: the fit depends on the cut, the elasticity of the material, and the design of the garment.
How thermal underwear should fit
The base layer is in contact with your body, so the fit should be fairly snug. There should be no large folds at the stomach, back, or legs. But the fabric should not be stretched so tightly that it looks like the set is a size too small.
A simple check takes 2–3 minutes. Raise your arms above your head, lean forward, do a few squats, and bend your elbows. The T-shirt should not ride up too far above the waist, and the seams should not dig into your underarms.
With thermal pants, check a deep squat and a wide stride. If the waistband slips down, the fabric pulls tightly across the knees, or you feel pressure in the groin area, it is best to try the next size.
This is especially noticeable during active use. Imagine walking briskly with a backpack for 2 hours: the shoulder straps press the clothing against you, your torso is constantly moving, and you have fleece and a jacket on top. A poor fit that seemed “almost okay” at home will become irritating on the trail.
If your measurements fall between two sizes
This happens often. For example, your chest circumference may correspond to one size, while your waist or hips are closer to the next.
Do not automatically choose the smaller size for a tighter fit. First, check exactly where the boundary lies. For example, if the T-shirt fits well around the stomach but pulls sharply under the arms when you raise your arms, you do not need to get the smaller size. If the larger option bunches up in large folds at the back and sags away from the chest, it is already too roomy.
What you need here is not a “nice-looking number” on the label, but a proper fit when moving.
Why you should not size up
For a hoodie or a casual T-shirt, a few extra centimeters may be part of the intended fit. The base layer works differently.
If a thermal T-shirt hangs loosely at the waist and the sleeves bunch up like an accordion, this fabric will start to shift under another layer. Add a fleece sweater and a jacket — now you have 3 layers, and extra volume between them certainly does not help.
During active movement, a loose set may twist at the waist, bunch up under backpack straps, or ride up with the fleece.
That is why we do not recommend choosing XL instead of L just “to be sure it does not constrict you.” If a specific size really restricts your movement, it is better to switch to a larger size after trying it on or checking your measurements, rather than adding extra room in advance.
A smaller size does not offer any advantage either
A snug fit does not mean maximum compression.
If after 10–15 minutes the waistband leaves a deep mark, the sleeves constrict your forearms, or the fabric across your chest is stretched to its limit, the set is too small. There is no point hoping that it will “stretch out.”
This becomes especially apparent in situations involving a lot of torso movement: working with gear, winter training, or climbing uphill with a backpack. Everything may look fine when you are standing still, but after ten leans or squats, the fabric will start to pull.
Thermal underwear should move with you, not force you to adapt to it.
A check before buying takes a few minutes
If you are trying on a set in a store, do not limit yourself to the mirror. Do a quick mobility test:
- do 5 squats and several forward leans;
- raise your arms above your head and bring them together in front of you;
- walk around and take a wide stride;
- put on the fleece or jacket you plan to wear with the set.
After this, it will be much easier to tell if the size is right.
If some of your measurements fall into one size and others into the next, write to our manager before placing your order and provide your measurements. We will help you compare them with that specific model and its fit so that you do not order thermal underwear at random. You can find the size charts for the models in the men's thermal underwear and women's thermal underwear sections.
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