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Backpack Size Capacity Explained

When choosing a backpack, whether it’s for day hikes, extended trips or somewhere in between, it can be difficult to know what size capacity you need. To make matters more confusing, different backpack manufacturers use differing methods to measure the capacity, or volume, of their packs. As a result, you may see capacity expressed in terms of liters or cubic inches (and perhaps both!) on specification charts. In this informative Expert Advice video, Gear Specialist Becky sheds a little light on the sometimes mystifying subject of backpack capacities and offers some tips on what to look for when determining the size capacity you need.

What Size Pack Do You Need?

Units of Measurement

The number that’s generally part of a backpack’s model name, regardless of manufacturer, most commonly references that pack’s capacity in liters. Why? Because the Osprey Tempest 16, for example, is a better name than the Osprey Tempest 976.38, which is that same pack’s capacity in cubic inches! But seriously, there are pros and cons to expressing pack capacity in either unit of measurement, and backpack manufacturers take that fact into account when designing, and later, providing specifications for individual packs.

Nalgene bottle to demonstrate 1 liter size
Picturing the capacity of a 1-liter Nalgene bottle can be a useful tool to determine backpack capacity.
  • Capacity expressed in terms of Liters: When backpack capacity is measured and expressed in terms of liters, it’s important to remember that the measurement is often rounded to the nearest, cleanest number, and so may not be exact. It is a good guide, though, and provides an easy visualization trick: picture a one-liter Nalgene bottle dumping its contents into the pack. However many Nalgene bottle refills it would take to fill the pack to the top (if it were water-tight!) is that pack’s capacity. (So, that Osprey Tempest 16 could hold the contents of a Nalgene bottle refilled 16 times…probably don’t try this trick at home with your new pack).
  • Capacity expressed in terms of Cubic Inches: When backpack capacity is measured and expressed in terms of cubic inches, you will sometimes be getting a more exact capacity measurement compared to measuring in liters. Some pack manufacturers provide this measurement as a matter of course, but not all do. You can obviously convert a liter measurement into cubic inches, but that may not be as accurate as if the manufacturer also provides the measurement.

Matching Capacity to Specific Uses

If you’ve narrowed in on a brand and model of pack, you’ll notice that most manufacturers have a range of capacity options available for that specific model (and perhaps Men’s and Women’s specific sizing, as well. More on that in our Why Choose a Women’s Specific Backpack? video). For example, the Deuter ACT Lite pack is available in a 40, 50, 65 and 75 liter capacity. All of the capacity options in the ACT Lite model line have a shared feature set, but are simply designed to hold different amounts of gear for different backpacking scenarios.

As a general rule of thumb, capacity ranges are matched to specific planned uses as follows:

Collage of backpack names to demonstrate size variation
Numbers at the end of a backpack's name usually refer to its volume capacity in liters.
  • Daypacks: Up to 40 liters
  • Overnight packs: 30-50 liters
  • Weekend packs: 40-70 liters
  • Extended trip packs: 70 liters and up

Personal Packing Style Is the Most Important Consideration

Backpacks of various sizes next to each other
Backpacks come in many sizes. Make sure you choose one that works for your needs!

In the end, how you personally plan to pack and travel is the most important consideration and you don't need to confine yourself within a specific capacity range. If your head is spinning thinking about all of the various backpack capacity options and you’re still not sure how to choose, just remember not to get hung up on the labels like “overnight” or “weekend.” Instead, focus on the specific gear that is going to end up inside of your personal pack. How many different things you plan to carry and the space required to carry those things can be more important than how long you plan to be out in the backcountry with those things.

Maybe you’re an ounce-counting minimalist who plans to pack light for an extended trip? It’s possible you would be happier carrying a weekend pack or even an overnight pack.

Or, maybe you’re an overnight backpacker who wants to be ready for anything in the great outdoors (meaning, you tend to bring everything including the kitchen sink along with you!). You may want to consider traditional extended trip packs in order to accommodate all of your gear.

Families or groups who are backpacking together and splitting up the weight of their gear may want to look at different size capacity packs even though they are all on the same trip. Each individual may end up with a different size pack, as one person may be carrying more of the load than another (for example, parents backpacking with their children are most likely going to be carrying more of the group’s camping gear, such as the tent).