In plain English
A liter is a unit of space, the same one printed on soda and water bottles. When a backpack says 30 liters, it means you could fill it with roughly thirty one-liter bottles' worth of stuff, counting the main bag and most of its pockets.
The number tells you how much room there is, not how heavy the pack is or how much weight it can carry comfortably. A 60-liter pack stuffed with a fluffy sleeping bag might be light. The same pack full of water and food would be very heavy.
Liters are handy because nearly every brand uses them, so you can compare packs quickly. Just remember that brands count pockets in slightly different ways, a bit like two apartments listed at the same size where one counts the balcony and the other does not. It is worth checking how much space is in the main bag too.
Liters are the volume unit hiking backpacks use to describe how much gear they hold. Manufacturers calculate it by adding up the main compartment plus most or all external pockets — side water-bottle pockets, the front mesh or shove-it pocket, and sometimes the lid or brain pocket, depending on the brand's measurement method.
Liters describe potential storage, not the pack's full carrying ability. Items lashed to the outside — a sleeping pad under compression straps, trekking poles in a side pocket, an ice axe on a loop — add carrying capacity without changing the printed liter figure. Two packs both listed at 50L can also differ slightly in real usable space depending on pocket shape and how the manufacturer counts external storage, since there's no single universal industry standard for exactly which pockets get counted.
Older packs and some outdoor gear sold in the US also list capacity in cubic inches rather than liters; one liter equals roughly 61 cubic inches, so a 65-liter pack is close to 4,000 cubic inches. Most current hiking-backpack listings use liters, but it's worth converting when comparing an older pack or a hunting-specific model that still uses the imperial unit.
Because liters are the standard unit across nearly every hiking-backpack brand, they're the fastest way to compare packs from different makers at a glance, even before checking torso fit, frame type or specific features like hipbelt padding or hydration-sleeve compatibility. Start any pack search by narrowing to a liter range that matches trip length, then compare fit and features within that shortlist.
Volume is not weight
Liters describe space; weight comes only from what you put in. Water is the handy reference. Its density is about 1 gram per milliliter, so a liter of water has a mass of about 1 kilogram.1 That is no coincidence: the original metric kilogram was defined as the mass of one liter of water.2 A two-liter reservoir therefore takes two liters of space and adds about 4.4 pounds.
Weight capacity is a separate rating set by the frame and hip belt. REI says its 55-liter Flash is comfortable with loads up to 35 pounds,3 while ULA gives its 55-liter Ohm a 30-pound maximum load.4 Same volume, different limits.
What gets counted
The page notes there is no single industry rule for which pockets count. Brand specs show why that matters. Gossamer Gear lists its Mariposa 60 as 60 liters in total, with 36 liters in the main body and 24 liters across seven exterior pockets.5 A pack that counts mesh and hip-belt pockets can post a bigger number than a pack with a roomier main compartment and fewer pockets. When the main body matters, such as fitting a bulky sleeping bag or a bear canister, look for a main-body figure or compare the packs' dimensions.
Gear lashed outside the pack is not counted either. Compression straps, which the Appalachian Mountain Club advises cinching once everything is packed to shrink and stabilize the load,6 often double as the attachment points for a sleeping pad.
Converting cubic inches and other units
A liter is about 61 cubic inches, so dividing a cubic-inch figure by 61 gives liters. The conversion is useful beyond older packs. Glacier Bay National Park lists its loaner bear canister as 560 cubic inches,7 which works out to about 9 liters that must be reserved inside the pack.
Airline size limits work the same way. Delta's carry-on maximum of 22 by 14 by 9 inches,8 multiplied out, is 2,772 cubic inches, roughly 45 liters of box. A soft pack of that volume may still bulge past those exact dimensions when full, so check the pack's measured size as well as its liters.
