Ski backpack capacity is measured in liters, and the right size depends entirely on how long you'll be out and what you need to carry. For half-day resort laps or short backcountry tours, 15–22 liters is plenty—just enough for water, snacks, an extra layer, and your avalanche safety gear. Full-day tours call for 25–35 liters so you can fit additional food, layers, and maybe a thermos.
If you're planning overnight hut trips, look at 36–45 liters to accommodate sleeping gear and extra supplies. Multi-day expeditions may require 46 liters or more. The golden rule: choose the smallest pack that fits your needs. An oversized pack tempts you to overpack, and excess volume leads to shifting loads that throw off your balance on skis.
Don't forget to account for avalanche gear—your shovel and probe take up dedicated space, and that's non-negotiable. Many skiers underestimate how much room layers and safety equipment require together, so size up slightly if you're unsure.