Thick, and tall enough
Mountaineering boots rise high and flex little, so the sock needs a cuff well above the boot: Darn Tough’s Mountaineering Over-the-Calf measures 17 inches (men’s) and 16.25 (women’s). Full cushion runs through the whole foot.
Size the boot with the sock
Mountaineering boots are usually fitted with a thick sock. Try on boots in the socks you will climb in; a thick sock in a boot fitted for a thin one squeezes the foot.
Liners under heavy socks
Some climbers wear a thin liner under a heavy sock so a damp liner can be changed without the outer. Try the whole system in the boot.
Dry socks overnight
Damp socks get cold. Keep a dry pair sealed for the evening, and dry the day’s pair inside your sleeping bag or jacket rather than near a stove.
What this can’t tell you
No sock has a temperature rating, and this guide has not tested any in the mountains. The list is chosen by cushion and cuff height; your boots, how much you move and dry socks matter as much.
