Backpacks by Capacity Questions, Answered
14 researched, fact-checked answers to the questions hikers ask most about backpacks by capacity.
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In plain English
A backpack's size in liters is simply how much it holds, the same way a milk jug is sold by volume. One liter is about the size of a large water bottle, so a 20-liter pack holds roughly twenty of them. The number says nothing about whether the pack fits your body. Fit is a different measurement, taken along your back from the bony bump at the base of your neck to the top of your hip bones.
Most questions on this page come down to one idea: longer trips need more stuff, and more stuff needs more room. A day walk needs water, snacks, a jacket and a few safety items. A night outside adds a tent, a sleeping bag, a sleeping pad and a stove. Each extra night adds more food.
Weight is the other half of the story. Hikers use two numbers. Base weight is everything you carry except food, water and fuel, which get used up along the way. Skin-out weight is everything, including the clothes and shoes you have on. Think of base weight as an empty car and skin-out weight as the same car with passengers and a full tank.
Why liter ranges differ from source to source
The day-hike and weekend ranges in these answers (18–22 L for a typical day pack, 35–50 L for a weekend) are editorial estimates, not a published standard. The Washington Trails Association, a large hiking nonprofit, draws the lines a little higher: 25–30 L for a full-day hike, 45–50 L for a standard one- or two-night trip, 50–70 L for three to seven nights, and 70 L and up for big loads.1 Neither set is wrong. They describe different kits. A hiker with a small down quilt and a one-person shelter fits a night into far less space than someone with a synthetic bag and a two-person tent.
It also pays to check what a brand counts in its liter figure. Gossamer Gear lists the Mariposa 60 as 60 L in total, made up of 36 L in the main body and 24 L across seven outside pockets.5 Two packs with the same headline number can have very different main compartments, and bulky items like a sleeping bag only go in the main body.
Rigid and heavy items set the real minimum
On many routes the bear canister, not the trip length, decides the pack size. Glacier Bay's loaner canister holds about six days of food for one person and weighs 2.8 lb.3 Denali's small container weighs 3 lb and holds three to five days of food; the larger one weighs 5 lb and holds seven to ten days.6 New York's DEC describes typical canisters as about 8 by 12 inches.7 A rigid cylinder that size wastes space in a small pack, which is why the answers above flag canister routes as a reason to size up. Rules are park-specific and change: Rocky Mountain National Park, for example, requires a canister for every wilderness overnight, so check current rules for your route.8
Water is the other dense load. A liter of water has a mass of about 1 kg, so three liters adds about 6.6 lb.9 Carrying capacity also has a weight ceiling that is separate from volume. REI rates the Flash 55 as comfortable to 35 lb and the Traverse 60 to 45 lb, while Gossamer Gear gives the Mariposa 60 a 30 lb comfort load.10,11,5 A big pack only helps if its frame can carry what you put in it.
Base weight, skin-out weight and what the definitions leave open
No standards body defines base weight. The working definitions come from the Appalachian Trail Conservancy and from pack makers, and they agree on the core idea: all your gear minus consumables such as food, water and fuel.12,4 Gossamer Gear sets the common tiers: under 20 lb is lightweight, under 10 lb is ultralight and under 5 lb is superultralight.13 The base-weight answer here also leaves out clothes you wear and poles in your hand. That is a widespread hiker convention, but the definitions in these sources do not spell it out, so when you compare gear lists, check whether worn items were counted. Skin-out weight has no published definition in the sources checked; it is community shorthand for everything on you.
The 50 L answer assumes roughly 1.5 to 2.5 lb of food per person per day. Treat that as a hiking rule of thumb. No official backpacking figure was found; the best-known published energy numbers, 3,400 to 4,300 kcal a day, come from military research on cold-weather field work and are not a hiking standard.14
