By Dana Okoro, Editorial Reviewer · Published September 14, 2026, 6:03 PM ET
Direct Answer
A 40L pack suits efficient packers on one to two-night trips, while a 50L pack gives more margin for two to three nights or bulkier gear. Neither is universally better — the right choice depends on trip length, gear bulk and how tightly you pack, with 50L offering more forgiveness for beginners still dialing in their kit.
In plain English
The difference between a 40-liter and a 50-liter backpack is ten liters, about the space of five large soda bottles. That may not sound like much, but on an overnight trip it can decide whether your sleeping bag, jacket and food fit inside or end up strapped to the outside.
A 40-liter pack works well if your gear is small and light and you are good at packing tightly. A 50-liter pack gives you breathing room, which helps when you are new, when nights are cold and you need warmer clothes, or when you must carry a hard food container in bear country.
There is a catch with bigger packs. Extra room tends to get filled, like a larger closet slowly filling with clothes, and a heavier pack is harder work on every hill. The best test is to lay out all your gear and see which size it actually needs.
Choosing between a 40L and 50L pack comes down to trip length and packing efficiency rather than one size being objectively better. A 40L pack works well for one to two-night trips with a compact three-season kit, and it rewards efficient packing since there's little room for excess. A 50L pack comfortably handles two to three nights and leaves margin for bulkier items like a synthetic sleeping bag, a bear canister, or extra layers for shoulder-season weather.
Beginners often do better starting with the 50L option, since the extra volume forgives an inefficient pack job while gear choices and packing habits are still developing. Experienced hikers with a dialed-in ultralight or lightweight kit frequently prefer the 40L size even for slightly longer trips, since a smaller pack sits closer to the back and doesn't invite overpacking.
Factor
40L
50L
Typical trip length
1–2 nights
2–3 nights
Best for
Efficient/lightweight packers
Bulkier gear, beginners
Overpacking risk
Lower (less room to fill)
Higher (more room to fill)
Weight is worth weighing alongside volume. A 50L pack's frame and materials are typically built to carry a somewhat heavier load than a 40L pack in the same product line, which matters if a hiker's gear list includes heavier items like a canister stove system or extra camera gear rather than a stripped-down ultralight kit. A pack that's comfortable loaded to 30 pounds may feel notably different at 40 pounds even if both liter sizes technically have room for the gear.
The better test than liters alone is whether your actual gear — shelter, sleep system, food and layers for the trip — fits with the main compartment reasonably full but not straining the zippers or compression straps. Laying out a full gear list before buying, rather than guessing from trip length alone, is the most reliable way to decide between the two sizes.
Where the two sizes sit in published guidance
The Washington Trails Association suggests a 45 to 50 liter pack for a standard one- or two-night trip and 50 to 70 liters for three to seven nights.1 By that yardstick, 50 liters is a standard overnight size and 40 liters is on the compact side, workable when the kit is light and small. This page's split, 40 liters for one to two nights and 50 for two to three, assumes somewhat more compact gear than WTA does.
Base weight, meaning the weight of your gear minus food, water and fuel, is the best predictor of which size you need. Gossamer Gear defines lightweight backpacking as a base weight under 20 pounds and ultralight as under 10 pounds.2 Kits near the ultralight line are the ones most likely to fit 40 liters for a couple of nights; heavier traditional kits usually need 50 or more.
The bear canister test
If your trips go where hard-sided food containers are required, the canister can make the decision for you. Glacier Bay's loaner canister holds 560 cubic inches, about 9 liters, and weighs 2.8 pounds.3 That is close to a quarter of a 40-liter pack but under a fifth of a 50-liter pack, and a rigid cylinder leaves awkward gaps around it. Rocky Mountain National Park, for example, requires canisters for all wilderness overnights,4 so check the current rules where you plan to hike.
Volume, load rating and effort
The page notes that larger packs are often built for heavier loads. Ratings vary widely even among similar sizes. ULA gives its 55-liter Ohm a 30-pound maximum load,5 REI says its 55-liter Flash is comfortable with loads up to 35 pounds,6 and REI rates the aluminum frame of its 60-liter Traverse for loads up to 45 pounds.7 If your loaded weight will be near 30 pounds or more, compare load ratings before you compare liters.
Carrying more has a measurable cost. A study of walking mechanics found metabolic energy use rose roughly linearly with backpack load, by about 7.6 watts per extra kilogram.8 UK infantry testing with heavier loads found metabolic cost rose about 1% for every additional kilogram carried.9 Ten spare liters are harmless on their own; ten liters filled with things you did not need is what you feel on the climbs.
