Alex's quick reference guide

Most of the time, I'm not sure.

You'll see a lot of backpacks that are suspiciously compact for their rated capacity. To my knowledge, there is no law, regulation, or industry standard anywhere in the world that requires a truthful statement of capacity. There is a testing standard, known as ASTM F2153-07(2018), but it’s not mandated by any government.

A tiny number of manufacturers will perform this test. But Mystery Ranch has quit the civilian market, and I can’t think of any larger manufacturers that might provide verified capacity for all of their pack products. Smaller companies do: Evergoods and Tom Bihn come to mind. Two photography-specific backpack designers also always have honest specifications: Lowepro and Think Tank.

Otherwise, for most manufacturers, the capacity stated in the model name or the specifications should be considered illustrative of the design intent, and not the true capacity. The real capacity is probably between 5% to 30% lower than the implied capacity in the model name or listed specification.

The method for measuring capacity is to fill a bag with ping-pong balls, then pour the balls into a cylindrical container with marked capacity indicators. However, by overstuffing all the pockets, and cramming as many balls as possible into the open-topped mesh water bottle pockets, sometimes manufacturers end up giving an inflated specification.

Even worse, sometimes they will measure according to a simple multiplication of the linear dimensions (L x W x H) and wildly overstate the capacity. Even trusted companies like Osprey will do this occasionally! It is very difficult for most people to verify manufacturers’ claims, because having most people don’t have 100L of ping-pong balls and a giant plastic cylinder kicking around at home.

Sometimes it's the opposite, and the capacity may be overstated. However, this situation is extremely rare, and to my knowledge, of the most popular models only the Cotopaxi Allpa 35 and 42 are like this (to their detriment, because it will no longer fit in the carry-on sizer when filled to its true capacity).

The good news is that most travel backpacks are, by virtue of their rectilinear designs, easy to calculate the likely capacity of. The Patagonia Black Hole MLC 45 is almost certainly really 45L, the MEC Outpost 30 and Patagonia Black Hole Mini MLC 30 almost certainly 30L, and so on (This is also how we know the very popular, affordable, and versatile Osprey Daylite Carry-On Expandable 26+6 is really closer to 20 litres + 4 litres). Alpine-style backpacks with large extension collars and floating top lids can also be easily overstuffed beyond its rated capacity.

Some backpack designs reduce practically usable volume in a way that cannot be attributed to deception or carelessness. This usually happens with backpacks with metal frames that have a 3D curvature, especially those with trampoline-style ventilation harness systems. The volume in the extreme corners is essentially ‘lost’ because flat, rigid items will block those spaces off.

Conversely, this makes trampoline back panels less desirable for air travel or for any situation where capacity needs to be maximized in a restricted rectilinear space (such as in a car's trunk, or under the airplane seat). Trampoline back panels, combined with the ergonomically curved shape of the packbag, will mean that a pack with a low capacity tends to have a large rectilinear footprint.

In sum, the listed capacity in the model name or the spec sheet is more indicative of a backpack's design goals and intended use case, rather than the true capacity. Therefore, the rest of this guide takes the model name's suggested litre capacity at face-value but when there's some serious chicanery involved I will do my best to note it. The North Face can't keep getting away with this!