What the litres on a backpack really tell you
The litre figure on a backpack is the brand’s own estimate, and brands measure it in different ways. Two bags with the same outside size can be sold as 15.5 litres and 32 litres. Among the 46 backpacks in our catalog sold as exactly 40 litres, the box their published dimensions make ranges from 26 to 76 litres.
Use the litres to compare bags from the same brand. To compare across brands, or to know whether a bag fits an airline, use the dimensions.
Stated litres against the box
For each of our 1,244 backpacks, we multiplied the three published dimensions to get the volume of the smallest box the bag fits in, then compared it with the litres the brand states.
| Stated volume compared with the box | Backpacks |
|---|---|
| More than 10% above the box | 261 (21%) |
| Within 20% below to 10% above | 644 (52%) |
| More than 20% below the box | 339 (27%) |
The median backpack claims 95% of its box. That makes sense: a backpack is rounded, padded and tapered, so it holds less than the rectangle around it. A rolling suitcase claims 82% of its box at the median, because wheels, handle and frame take space.
The surprising group is the first one. A bag that claims more than its box can hold is not measuring the same way. Usually the litres include things the dimensions leave out: an extension collar on a roll-top or a hiking pack, stretch mesh pockets, a lid that rises when full. Sometimes the dimensions are taken on the empty, flat bag. Either way, the number on the label is not the space inside the shape you see.
Same box, different numbers
A few examples from our catalog, each with a box of about 25 litres:
| Backpack | Size (cm) | Stated volume |
|---|---|---|
| Côte&Ciel Venta | 47 x 38 x 14 | 15.5 L |
| NIID Decode Backpack 25L | 46 x 32 x 17 | 25 L |
| The North Face Borealis | 50 x 30 x 17 | 28 L |
| Shimoda Sidecountry 32 | 51 x 29 x 17 | 32 L |
These four bags take roughly the same room in an overhead bin. The Shimoda is sold as having twice the capacity of the Côte&Ciel.
And at the 40-litre mark, the extremes are the ULA Equipment UltraGrid Nexus, 54.6 x 29.2 x 16.5 cm (a 26-litre box), and the Lowe Alpine Sirac Plus 40, 75 x 36 x 28 cm (a 76-litre box). Both say “40L”. The first fits the carry-on limit of 35 of our 36 airlines. The second is a hiking pack with a frame and hip belt and fits none of them.
Brands lean one way or the other
The ratio is fairly consistent within a brand, which suggests each brand has its own method:
| Brand | Backpacks | Median stated volume as a share of the box |
|---|---|---|
| Gossamer Gear | 16 | 146% |
| Cabin Zero | 17 | 125% |
| WANDRD | 15 | 116% |
| Patagonia | 18 | 107% |
| Tortuga | 7 | 99% |
| GORUCK | 13 | 97% |
| Aer | 17 | 89% |
| Tom Bihn | 23 | 87% |
| Nomatic | 11 | 82% |
| Osprey | 27 | 63% |
Gossamer Gear makes ultralight hiking packs with big mesh pockets and extension collars, and counts them. Osprey’s range is full of framed packs with harness and hip belt, whose outer dimensions include a lot that isn’t storage. Neither is wrong. They are answering different questions.
So a 35-litre bag from one brand can carry more than a 40-litre bag from another. When you compare across brands, look at the dimensions and at photos of the main compartment.
What this means when you choose
For airline fit, ignore the litres. Airlines check dimensions, and a “40L” bag can be fine or hopeless depending on its shape; see the biggest backpack that fits every airline.
For capacity, compare bags from the same brand. A brand’s 30L and 40L bags are presumably measured the same way, so the gap between them means something. Also check what the number includes: expandable bags often quote the expanded figure. 121 backpacks in our catalog are expandable, and their pages show both numbers.
A quick sanity check works for any bag: multiply the three dimensions in centimetres and divide by 1,000. If the stated litres are well above that, the brand is counting something outside the main compartment.
How we counted
Stated volumes and dimensions are the brands’ published figures, as recorded on each bag page with its source. The box is height x width x depth. We don’t measure bags ourselves, so this compares two of the brand’s own numbers, not a lab test. Our weight vs volume chart uses stated litres, with the same caveat.