Thermal Base Layer Sizing: Snug, Never Tight

A thermal base layer only works if it stays in contact with your skin — that is how it wicks moisture and reflects heat back. So the right fit is close, following your body without pinching. If it balloons away from your torso, warm air escapes and the fabric cannot do its job. Black Feather base layers run true to size. If you are between two sizes, the call depends on how you train and dress: size down if you want a compressive, next-to-skin feel or wear it for high-output activity; size up if you intend to stack a thicker mid layer over it or prefer a little more room through the shoulders and chest. When in doubt, favour the closer size for the top and bottoms separately — many people mix a size across pieces to fit their own proportions. A base layer that fits well should be the last thing you notice about it.