Backpack capacity is measured in liters, and the right number depends almost entirely on how long you are out. Pick too small and your gear will not fit; too big and you will fill the extra space with weight you do not need. Here is the simple sizing map, plus the thing that matters even more than liters.
Liters by trip length
- 15-30 L (day hikes): water, snacks, a layer, and the ten essentials. A tactical-style daypack like the REEBOW 45L pack (4.7 stars, 17,000+ ratings) also doubles as an overnight bag.
- 30-50 L (overnight / weekend): the big three plus food for a night or two.
- 50-65 L (multi-day): the weekend-and-beyond standard. A Kelty 55L backpacking pack (4.7 stars) sits right in this sweet spot.
- 65 L+ (week-long / winter): bulky cold-weather gear, extra food, more water capacity.
Fit beats capacity
A pack that matches your torso length and rides the weight on your hips will out-carry a bigger pack that fits poorly every time. Measure your torso (the distance from the bony bump at the base of your neck to the top of your hip bones) and check the pack's size range before you buy. Most weight should sit on the hip belt, not the shoulders.
Pack for the trip, not the pack
A bigger bag is a bigger temptation to overpack. Size to your longest common trip, then compress it down for shorter ones. Add water capacity and a treatment method for dry routes (a UV purifier or filter weighs little and earns its place).
See how the pack anchors the rest of the kit in your first backpacking kit, or browse packs in camping.
As an Amazon Associate, Limit Outfitter earns from qualifying purchases. We link to gear we would run ourselves; check current pricing and availability on Amazon.