Backpacks comes in all shapes and sizes measured by length, width, and height. Seems simple enough. To get volume, just multiply those three sides up, right? Well, it turns out a very common backpack measurement, liters, is one that varies based on who’s calculating. You might see two backpacks that are listed as 34 liters but when you get those bags in hand, they might not have the same carrying capacity. There are a few reasons for this, related to more common units of measurement.

You can learn more about how backpack liters are calculated in the video above or read on.

Basic Volume

We know that length multiplied by width and height equals volume. But that’s volume of a cube and most backpacks aren’t completely square. They’ve got rounded corners, so height might be to the top of the bag without accounting for space lost to curved edges. (Length width and height measurements are made by the way when the bag is completely full, potentially stuffed, to give the best numbers.)

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Some backpack manufacturers might try to get away with this to boost their capacity numbers but another, more hands on approach is often used.

Fill The Bag With Balls

To determine the capacity of a backpack small pellets are used to fill every last nook and cranny to determine volume. Fill up the bag, then measure how many liters of pellets you can fit. Always keep that in mind when reviewing backpack specs. The entire usable space isn’t likely to be used unless you’re traveling with a backpack full of sand. Otherwise the carrying capacity is an upper limit, not an average, and it’s measured under ideal conditions.

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Exactly how this is done isn’t standardized – although a lot of people use the word standard when talking about backpack liters – there’s no governing body or backpack liters organization issuing a set of specific guidelines that everyone follows. Backpack companies use all sorts of methods to measure their bags.

Literal Variation

A lot of companies only measure pockets and compartments that can be closed with a zipper while others include side compartments, water bottle holders, and other open pockets in their liter measurements. This is a pretty grey area because it is technically storage space and companies are trying to get a capacity measurement as high as they reasonably can.

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The reason it’s a grey area isn’t because this is a shady practice, it’s because not every company is measuring things the same way, which can make things confusing for you, the consumer. Some companies have even gone away from using liter measurements for this very reason.

Understanding Liters

The best way to think about backpack liters is in generalities. They’re ball park figures, basically to give you an idea of what a backpack can carry which of course depends on the configuration of the backpack and what you’ll be packing – clothes are easier to stuff in a bag than camera gear – so my advice to you is to think of backpack liters ranges like this:

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These though are just general guidelines. You know now that liters can vary – a 25 liter backpack by one brand doesn’t’ necessarily have the same capacity as a 25 liter bag from another brand. Fortunately though most companies give you enough of a return window where you can try out a bag, pack it up, and do your own, customized capacity test. A fitting process you should go through so you can pick the right backpack for you.