What Size Backpack Do You Need?

Steven displaying three Osprey backpacks on tree branch

When choosing a backpack, whether it’s for day hikes, extended trips or somewhere in between, it can be difficult to know what size you need.

To make matters more confusing, different backpack manufacturers use differing methods to measure the capacity, or internal space, of their packs. As a result, you may see capacity expressed in terms of liters or cubic inches – or perhaps both – on specification charts.

We’ll offer some tips on what to look for when determining the size capacity you need.

Units of Measurement

Backpacks often include a number as part of their name. This commonly references the 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.


When backpack capacity is measured in liters, remember that the measurement is often rounded to the nearest 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. For example, you could in theory pour 16 Nalgene bottles into our Osprey Tempest 16 liter pack (though we don’t suggest it).

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 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 packs to people

If you’ve decided on a brand and model of pack, look to see if they offer women’s or youth-specific packs. These packs are designed to provide a better fit for women and smaller hikers.

matching pack to purpose

Most manufacturers have a range of capacity options available for a specific model. For example, a pack may be available in 20 liter, 35 liter and 50 liter sizes.

The right pack size for you is determined by the type and length of the adventure(s) you’re planning. Unlike dayhikes, overnight trips require room for a tent and sleeping bag. Longer treks require more clothes and food. As a general rule of thumb, you can use these capacity ranges to determine how big your pack should be:

How to measure Pack volume

Volume Calculator

Just fill in the dimensions of your box as shown in the video, and this handy calculator will estimate the pack size you need!

Individual Packing Considerations

In the end, how you personally pack is the most important consideration. Don’t get hung up on labels like ‘overnight’ or ‘weekend’.

Instead, seek a pack that fits your needs. Focus on the specific gear that’s going to end up inside of your pack. Often, the gear you plan to carry and the space required can be more important than how long you’re planning to be out.

For instance, someone hauling professional camera gear or child care supplies will need a larger pack than an ounce-counting minimalist with ultralight gear.



Questions? Talk to a gear specialist.