Hiking Backpack Size Guide: What Size Backpack for Hiking?

Buy a 65-liter pack for a three-mile afternoon loop and you’ll lug an extra two pounds of empty nylon up the hill. Go the other way and you’ll spend the trip lashing a sleeping bag to the outside of a 30-liter bag. Neither has to happen. Capacity is the easy half of the decision — the sizing chart below covers it — and torso fit is the half that people get wrong.

How Many Liters Do You Actually Need?

Capacity maps to trip length and gear bulk, and those two numbers set the range you should be shopping in. REI’s backpack buying guide runs the standard breakdown: 18–24 liters for a half day, 25–30 liters for a full day out, 35–45 liters when you need layers for shifting weather, 45–50 liters for a one- or two-night trip, 50–70 liters for three to seven nights, and 70 liters and up for long multi-day or mountaineering loads.

Here’s the practical rule underneath it: pick your range from the table, then think about what you carry that’s bulky. A bear canister, a winter synthetic bag, and a four-season tent eat space fast.

That 18–24 liter column is the most common overshoot. Most day hikers carrying a 40-liter pack on a morning trail are hauling empty space they paid for in pack weight. And the reverse error shows up on overnight trips: 45 liters sounds generous until a synthetic bag and two days of food go in. If you’re shopping that range now, our tested picks for budget hiking backpacks that hold up cover which models actually carry weight well at each size.

What Size Backpack for Hiking Based on Torso Length?

Backpack size is set by torso length, not your height — a 5’2″ hiker and a 6’1″ hiker can wear the same size pack. REI’s fitting guidance uses two landmarks: the C7 vertebra, the bump at the base of your neck when you tilt your head forward, and the iliac crest, the top ridge of your hip bones. The distance between them is your torso length, and every pack lists a matching torso range (usually something like 16–19 inches).

Measure it with a flexible tape or a friend and a ruler, then match that number to the pack’s sizing chart before you look at colors or pockets. Get it right and the hipbelt lands on your hip bones, where it can carry the load — which matters more the heavier the pack gets.

The order of operations is worth repeating: trip length and gear bulk first, capacity range second, torso size and hipbelt fit third. Skipping to step three because a pack is on sale is how people end up with a bag that fits their back and not their trip.

Winter, Gear Bulk, And The Mistakes That Cost Money

Cold weather changes the math. Winter backpacking generally means sizing up, because insulation layers compress poorly and take real volume — a load that fits in 50 liters in July can push past 60 in January. The 50–70 liter range covers most three- to seven-night trips, but bulky winter gear can sit at the top of that window or beyond it.

Three mistakes account for most bad buys. Confusing height with torso length sends people to the wrong size entirely. Underestimating food, water, and a bear canister turns a “one night” trip into a puzzle. And buying for the conditions you hope for rather than the ones you get leaves no room for the extra layer you’ll be glad you packed.

A pack that’s too small forces the same bad outcome every time: gear strapped to the outside, where it swings, catches brush, and shifts the load off your hips. If a capacity range feels borderline for your gear list, go up one size rather than out.