Compare 28L and 40L hiking backpacks by capacity, empty weight, and frame type to find the right size for your trip length, gear list, and pace.
By Marcus Hale, Lead Pack Research Editor · Published September 14, 2026, 6:03 PM ET
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In plain English
A liter is a measure of space, like the size of a big water bottle. A 28-liter pack holds about 28 of those; a 40-liter pack holds about 40. The bigger pack has room for the things you need to sleep outside: a tent, a sleeping bag and a pad to lie on.
The smaller pack is like a gym bag: perfect for a full day out with water, snacks, a rain jacket and some safety gear. The bigger pack is more like a weekend suitcase. It usually has a stiffer frame inside and a padded belt that sits on your hips, so heavier loads feel easier to carry.
If you mostly go out for the day, the smaller pack is lighter and more comfortable. If you often camp overnight, the bigger one saves you from cramming. People with very small, light camping gear can sometimes squeeze a night out of the smaller size.
A 28L pack and a 40L pack differ by roughly 12 liters of usable volume, which is the difference between a large day pack and a genuine one-to-three-night backpacking pack. Choosing between them comes down to trip length, gear volume, and whether you're packing for speed or for comfort margin.
How much bigger is a 40L pack than a 28L pack?
A 40L pack holds about 40% more volume than a 28L pack, enough for a three-season sleeping bag, a small tent or tarp shelter, and two to three days of food alongside the same clothing and safety gear a 28L pack carries for a day hike.
That extra volume typically goes toward:
A compressible three-season sleeping bag or 20°F quilt
A one- or two-person backpacking tent or tarp shelter
A sleeping pad, either inflatable or closed-cell foam
Two to three days of food instead of one
A change of camp clothing and extra socks
A 28L pack, by contrast, has just enough room for a day's food and water plus a shell layer, with little margin left for a sleep system unless the kit is unusually compact.
Which size fits day hikes and fastpacking?
A 28L pack fits day hikes and fastpacking better than a 40L pack because the smaller volume forces a lighter, more compressed load that rides closer to the back and doesn't shift during running or scrambling sections.
Fastpackers favor packs in the high-20s to low-30s liter range specifically because a half-empty 40L pack tends to sway and bounce, even with compression straps cinched down. A 28L pack with a vest-style harness, similar to the design used on the Osprey Duro and Salomon ADV Skin packs, keeps weight stable against the torso during faster movement.
Which size fits multi-day trips?
A 40L pack fits multi-day trips of two to four nights better than a 28L pack because it has room for a full sleep system, shelter, and several days of food without over-stuffing the main compartment or relying on external lashing for core gear.
Packs in the 40L range, such as the Osprey Exos 38, Gregory Zulu 40, and Deuter Aircontact Lite 40, use sturdier internal frames designed to carry loads through 30 to 35 pounds comfortably. A 28L pack pushed to that same weight loses stability, since its frame and hip belt are built for a lighter carry.
How to decide between 28L and 40L
Match pack size to the longest trip type you take regularly rather than the average one. Hikers who mostly day hike but take one or two overnight trips a year often do better with the smaller 28L pack and a genuinely minimal overnight kit than with a 40L pack that sits half-empty most of the year.
Consider these questions before choosing:
What is the longest trip, in nights, you take more than once a season?
Does your sleep system compress small enough to fit a 28L pack?
Do you prioritize speed and low weight over comfort margin?
Will the pack also serve as a travel or commuter bag?
A hiker who answers "day hikes and one fast overnight" to the first question usually does better with 28L. A hiker planning regular two-to-four-night trips gets more consistent comfort from 40L, even if it means carrying a slightly heavier empty pack on shorter days.
Weight difference between the two sizes is smaller than the volume gap suggests. An empty 28L pack often weighs 1.5 to 2.5 pounds, while a comparable 40L pack with a fuller suspension system typically runs 2.5 to 4 pounds. That extra pound mostly buys frame structure and hip belt padding built to carry the heavier loads a 40L pack is meant to hold, not just extra fabric for the bigger main compartment.
Some hikers solve the size question by choosing a pack with a roll-top or extension collar that adds several liters of adjustable volume on top of a base capacity. A 28L pack with a generous roll-top can flex toward 32-33L when needed, narrowing the practical gap with a fixed 40L design for hikers who only occasionally need the extra room.
Frequently Asked Questions
A 28L pack is small for backpacking but workable for one night with an ultralight kit: a compressible quilt, minimalist shelter, and canister stove. It's too small for standard three-season gear over multiple nights, where a 35-40L pack gives a more comfortable margin.
Not always. A 40L pack gives comfortable room for a two-night trip with standard gear, but hikers using an ultralight sleep system and minimal food can manage two nights in 30-35L. Bulkier winter or cold-weather gear makes the extra volume of a 40L pack more useful.
The Washington Trails Association says a 25–30 liter pack is good for a full-day hike, a 45–50 liter pack should fit gear for a standard one- or two-night trip, and 50–70 liters suits trips of three to seven nights.1 By that yardstick, 28L is a day pack with a little spare room, and 40L is smaller than the usual overnight range.
That doesn't make 40L wrong for the two-to-four-night trips the page describes. It means a 40L pack on those trips assumes more compact gear than WTA's baseline. If your sleeping bag and tent are standard rather than lightweight, expect a 40L pack to feel full on two nights and tight beyond that.
Base weight decides whether 28L can go overnight
Base weight is your pack's weight with all gear, minus consumables such as food, water and fuel. The Appalachian Trail Conservancy calls the pack, shelter and sleeping bag the 'big three', usually the heaviest items you carry.4 Gossamer Gear groups kits as lightweight under 20 pounds, ultralight under 10 and superultralight under 5.2 These labels come from gear makers and trail groups, not a standards body.
Weight and bulk usually travel together, and the sleeping pad shows it well. Big Agnes suggests an R-value of 2 or lower for summer, but 4 to 4.8 for spring through fall because shoulder seasons still bring cold nights.5 R-value measures resistance to heat flow, so a higher number means a warmer pad,6 and warmer pads are generally bulkier. A 28L overnight is far more realistic in midsummer than in October.
What the extra pound of frame buys
Research on walking with loads finds energy use rises roughly in a straight line with pack weight, by about 7.6 watts per extra kilogram in one Journal of Experimental Biology study.3 A military review adds that the cost also climbs with walking speed and slope.7 The 40L pack's extra empty weight is a real cost on every day hike, and the payoff only comes when the load is heavy enough to need the frame.
That payoff is weight transfer to the hips, and how much gets there is disputed. Big Agnes aims for about 80% of pack weight on the hips,8 Alpkit says 60–80%,9 while US Army doctrine estimates only about 30% on the hips with a frame and hip belt.10 Whichever figure is closer for you, a thin 28L belt can't carry much of a heavy load. Water is often what tips the balance: in dry country the National Park Service at Guadalupe Mountains advises a gallon per person per day, about 8 pounds.11
