The Comprehensive Guide to Backpack Sizes and Liters

Feeling a bit lost in the world of backpack sizes? Uncertain about how many liters your ideal backpack should be? This all-inclusive guide to backpack sizes and liters is here to demystify the process. Whether you're an experienced traveler or just starting out in the work world, check out this review to ensure that your next backpack purchase is a breeze.

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backpack sizes

Do you want a new backpack but are feeling overwhelmed by all the sizing jargon? You're not alone – backpack companies can make it feel like you need a degree in Industrial Design to choose the bag that’s right for you.

While it's easy to know, for example, that you want a laptop compartment in your new backpack, things get confusing when presented with the choice between a 2,500 cubic inch or a 40-liter laptop backpack (which are pretty much the same size, by the way). So, if you want some help unraveling the mysteries of backpack sizes, check out this handy guide before you browse online or hit the stores. It's your key to simplifying the backpack sizing puzzle.


Why Size Matters

Whether carrying your laptop and daily essentials to work or travelling with clothes for a week, your backpack can get filled pretty quickly with some heavy items. So having the right-sized backpack is critical to ensuring that you can comfortably haul all the stuff you need. For example, a backpack that is too large will be heavier than needed and is more susceptible to carrying too many “just in case” items which also add unnecessary weight.

Additionally, you don’t want a backpack that is too small to carry the things you absolutely need to have with you.


How Backpack Size Is Measured

Since size matters, it follows that being able to compare sizes among the huge number of backpacks on the market is important. But before diving into the differences between various backpack sizes, let’s get grounded in how backpacks are measured.

First, it’s essential to understand that when we talk about “sizing”, we’re talking about the three-dimensional space the backpack provides to carry things (i.e., its volume or capacity), not its exterior dimensions. This volume is usually expressed in one of two ways: either by liters or cubic inches. Even in the US, liters are the most popular way to describe the size of a backpack.

>>MORE: Convert liters to cubic inches

So far, so good. Now here’s where it gets tricky. The methods that companies use to calculate a backpack’s volume, whether in liters or cubic inches, are all over the map.

Calculating a backpack's size by multiplying its exterior height, width, and depth might seem like the simplest route, but here's the scoop: it's not really accurate. This method works okay only for plain rectangular bags like your standard grocery tote. But when you're dealing with the highly-functional backpacks that you take to work or on your travel adventures, it's like trying to fit a square peg in a round hole. The backpacks you’re looking at have external curves as well as interior pocketing, compartmentalization, and organization that eats up capacity. So, our advice is to ditch the high school math formula.

Most reputable brands (including Knack) measure a backpack’s volume by following the American Society for Testing and Materials (ASTM) F2153-07 standard. This standard requires all fully enclosed pockets and compartments to be filled completely with 20mm plastic balls and closed. The balls are then dumped out into a graduated cylinder and their volume is measured. The resulting measurement is expressed in either liters or cubic inches. Voila! An accurate way to measure backpack size. Another version of this approach, while even slightly more accurate, but much harder to do efficiently, uses water instead of the plastic balls.  

Okay, so if there’s an accurate standard used to calculate backpack volumes, shouldn’t all published sizes be comparable? Unfortunately, no. Some companies also include the volume of pockets/compartments that are not fully zipped (think exterior water bottle pockets, exterior compression pockets, shoulder strap pockets, etc.) in their final backpack sizing measurements. We believe that including the capacity of these types of pockets gives a misleading depiction of a backpack’s true capacity so Knack’s volume measurements always follow the ASTM standard to the letter.


Common Backpack Sizes by Use

Given the lack of uniformity around calculating backpack sizes in liters or cubic inches, we suggest that you should only use published volumes as a general guideline while searching for your next backpack. It’s more helpful to differentiate between backpacks by focusing on the bag’s functional features and how comfortable it is when you wear it.

While all this sizing information is great, you probably still want to know how much stuff that new backpack you’ve been eyeing will hold. Although we said you should only use the backpack’s published capacity as a general guideline, you can still answer that question by using the “t-shirt rule of thumb.” While not an exact science, the t-shirt rule of thumb can be helpful in translating backpack liters into something more tangible. Imagine the amount of space a rolled or folded men’s large cotton t-shirt takes up; you now know what a liter of clothing looks like. Furthermore, if you carry the most popular US laptop size for work, one with a 15.6-inch screen, you now know what around 2 liters of technology looks like.

>>MORE: The Best Size Backpack for Travel

In addition to using what goes in your backpack to help understand backpack sizes and liters, there is one other measurement to think about before you make your final decision: 45 liters. 45 liters represents the maximum size backpack that can be carried onto most airplanes in the US.

Finally, the chart below gives you a simple summary of popular carry-on backpack volumes and how people typically use backpacks of that size.

backpack small medium large

Backpack Size and Liters Chart

Carry-On Backpack Sizes


Cubic Inches


Daypack or Small Backpack

15 - 24 Liters

915 – 1465 cu inches

Exploring cities, everyday adventures, errands.

One-night trips.

Medium Backpack

24 - 35 Liters

1465 – 2135 cu inches

Work, carrying your laptop. Overnight trips of 2-3 days

Large Backpack

30 - 45 Liters

1830 – 2746 cu inches

Work (if you carry a lot of stuff or a large laptop). Overnight trips of 3-4 days.

"This is a great bag to use for everyday carry, as well as traveling without the hassle of transferring your daily carry items into another bag. The material is high-quality, the storage is well thought out, and the medium is just the right size for daily carry." - Brad B



The Benefits of an Expandable Backpack

Charts and rules of thumb are helpful, but there’s an even easier way to ensure that you buy the right size backpack: get an expandable backpack.

The capacity of expandable backpack, like a Knack Pack, is designed to grow and shrink based on whatever carry needs you have that day. As a result, the same expandable backpack can serve many different uses that require different sizes: work, travel, gym, and around town.

Knack Packs are lightweight, expandable, laptop backpacks that range in size from 15 liters (unexpanded) up to 46 liters (expanded).

>>MORE: Compare All Knack Backpack Sizes

Every Knack Pack sports a main compartment that is designed to carry all your everyday items in an organized manner so that they are always within reach. But what makes Knack expandable backpacks unique is their hidden, patent-pending suitcase section that expands when you need it and zips away when you don’t.
<Knack GIF expanding and contracting>

Check out this video to give you an idea of all the things that can fit in our most popular 27 liter Series 1 Medium Knack Pack when it is expanded to its full 39 liter size.

series 1 medium knack pack packing list

Understanding backpack sizes and liters can be a bit of a puzzle, but we hope this guide has shed some light on this often-confusing topic. Remember, when selecting your next backpack, it's not just about the numbers; it's about finding the right fit for your unique needs, whether it's for work, travel, or everyday adventures. And if you're looking for the ultimate in size flexibility, consider an expandable backpack, like a Knack Pack, which is designed to adapt to your changing needs.

Choosing the perfect backpack size doesn't have to be a confusing experience and Knack is here to make it easier for you.

"I travel a lot for work and my Knack Pack is perfect for the entire trip. Expand the bag for packing while traveling, collapse, and use for daily work once I arrive and looked great the whole time." - Dave A



Backpack Sizes and Liters FAQs

How are backpack sizes measured?

There are several ways to measure backpack size. The most common way is to fill the bag up with small plastic balls and dump the balls into a measuring device to determine the volume they represent.

Can I easily compare backpack sizes in liters across brands?

Unfortunately, you can’t. There is no consistency in how backpack companies measure the size of their bags. As a result, backpack sizes in liters or cubic inches should only be used as a general guideline, not an absolute one.

I need a backpack for work and to carry around town. How many liters should it be?

Assuming you don’t carry much to work everyday and have a small computer, you could use a 24-liter backpack. If you carry a 15.6-inch laptop and want room for just about anything you may need for work, you could use a backpack up to 35 liters.

What backpack size in liters can be carried on to an airplane?

Most US airlines will allow carry-ons up to 45 liters in size.

What’s the benefit of an expandable backpack?

Flexibility. An expandable backpack’s capacity is designed to grow and shrink based on your carrying needs so one bag can meet the same needs as several non-expandable backpack sizes.

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