By Dana Okoro, Editorial Reviewer · Published September 14, 2026, 6:03 PM ET · Updated September 29, 2026, 6:16 PM ET
Direct Answer
Most hikers need 15–25L for day hikes, 35–45L for an overnight, 45–55L for a weekend of two to three nights, and 55–75L for four-plus nights or winter loads. Match liters to trip length first, then adjust up for bulky gear like a bear canister or synthetic sleeping bag, or down for an ultralight kit.
In plain English
Backpack size is measured in liters, the same unit printed on a soda bottle. A 50-liter pack holds about as much as twenty-five two-liter bottles. The number tells you how much stuff fits inside, not how heavy the pack is.
The biggest factor is how many nights you will be out. A day hike needs water, snacks, a jacket and a few safety items. An overnight trip adds a tent, a sleeping bag and cooking gear, and each extra night adds more food.
Your gear matters too. Some gear is puffy or boxy and eats space, like the hard plastic food container some parks require in bear country. Newer, lighter gear often packs smaller. It is like choosing a suitcase: a weekend away needs less room than two weeks, but a winter coat can fill a small suitcase by itself. Once you pick a size, the pack still has to fit the length of your back.
The right hiking backpack size depends primarily on trip length, measured in liters of internal volume. A single day out needs 15–25L for water, layers and the ten essentials. An overnight trip needs 35–45L to add a shelter and sleep system. A weekend of two to three nights needs 45–55L. Extended trips of four to seven nights, or any winter trip, typically need 55–75L.
Gear bulk shifts these ranges. A hard-sided bear canister, a synthetic sleeping bag, or winter insulation all take up more space than their lightweight equivalents and can push a hiker up one capacity tier. An ultralight kit with a quilt and minimal cookware can do the opposite, fitting a longer trip into a smaller pack.
Trip type
Capacity
Day hike
15–25L
Overnight
35–45L
Weekend (2–3 nights)
45–55L
Extended (4–7 nights)
55–65L
Winter / week-plus
65–75L+
Capacity isn't the only fit factor, though it's the first one to nail down. Once liters are chosen, torso length — measured from the C7 vertebra at the base of the neck to the iliac crest at the top of the hip bones — determines which specific frame size within that capacity range actually fits. Two hikers buying the same 50L pack can need different frame sizes if their torso lengths differ, even at the same height.
Buy for the trip length you take most often, not the longest trip you might someday do — an oversized pack tends to get overpacked and carries less comfortably when only partly full, since the frame and hipbelt are tuned for a fuller load. If your trips vary widely in length, a mid-range 45–55L pack with solid compression straps is often more versatile than either a small daypack or a large expedition pack alone.
Why published size ranges disagree
There is no universal sizing chart, so guides draw the lines in different places. This page's table suggests 15 to 25 liters for a day hike and 35 to 45 for an overnight. The Washington Trails Association suggests 25 to 30 liters for a full-day hike, 45 to 50 for a standard one- or two-night trip, 50 to 70 for three to seven nights, and 70-plus for large loads.1 WTA's bands run larger at the short end, which leaves room for cold-weather layers and extra water.
Brands' own labels show the same spread. Kelty positions its 65-liter Asher for two to three days in the backcountry,2 while Osprey describes its Atmos AG 65 as built for heavyweight weekends or week-long trips.3 The same volume suits very different trips depending on what goes inside.
Bulky items that push you up a size
A hard-sided bear canister is the clearest example. Glacier Bay's loaner canister holds 560 cubic inches, about 9 liters, and fits roughly six days of food for one person.4 New York's DEC describes typical canisters as about 8 by 12 inches, holding up to five days of food.5 Because a canister is rigid, it takes up that space whether it is full or not. Canister rules are set area by area and change, so check the current rules for your destination.
Cold weather is the other big driver. The National Park Service's winter layering advice calls for a wicking base layer, an insulating middle layer and an outer layer that repels water and blocks wind,6 and carrying extra insulation for cold nights adds volume fast.
Volume is not the same as load capacity
A pack's frame and hip belt are designed for a certain weight, and that limit can be lower than the volume suggests. REI says its 55-liter Flash is comfortable with loads up to 35 pounds7 and rates the aluminum frame of its Traverse 60 for loads up to 45 pounds.8 Gossamer Gear gives its Mariposa 60 a 30-pound recommended comfort load and a 35-pound maximum.9 Dense items fill the weight budget faster than the volume: a liter of water has a mass of about one kilogram.10
The page advises against buying for the longest trip you might someday take, because a big pack tends to get filled. If you do choose a larger pack for flexibility, use its compression straps on shorter trips. US Army doctrine says compression straps keep the internal load from shifting and hold it closer to the body.11
