Confused about what’s the right travel backpack size for you? We've put together the ultimate backpack size guide to help you choose the ideal backpack size for your next adventure. Say goodbye to the hassle of overpacking or struggling with a too-small bag – find the right backpack size and enjoy stress-free travel!

What size backpack do you need for travel?

When you’re shopping for a backpack online, it can be really difficult to tell what size backpack you need for traveling - especially when backpacks come in such a huge range of sizes. You don’t have the bag in front of you, so how do you figure out which bag is going to fit all of your travel stuff?

In this guide, we’re going to help you figure out the backpack size that’s not too big and not too small, but just right for your travels.

Understand backpack sizing

Backpack capacity is almost always measured in liters - a universally accepted standard unit - which makes it much easier to compare backpack sizes across brands. As a rule of thumb, 25-30 liter backpacks are best for shorter weekend trips, while 30-45 liter backpacks are best for longer trips or long-term travel. That said, the capacity that you need depends on a number of different factors, which we’ll cover below. 

To visualize how much capacity is in a backpack of a certain number of liters, consider that a gallon jug holds about 3.7 liters. So a 20-liter backpack would have the capacity of about 5 and a half one-gallon jugs. A medium-size rolled-up t-shirt takes up about one liter of space, so you could fit about 20 rolled t-shirts in a 20-liter backpack. 

Keep in mind that backpack capacity doesn’t strictly correlate with backpack dimensions. While capacity tells you how much you can fit in your travel backpack, the backpack dimensions will give you a better idea of what you can fit in your backpack. Some 15-liter daypacks aren’t long enough to fit a 13” laptop, while some - like the Small Expandable Knack Pack - can do so. For this reason, it’s really important to consider the backpack’s volume and the backpack’s dimensions. 

How to choose the right backpack size for your travels

The destination, length, and purpose of your trip will all affect how much and what stuff you need to bring, and therefore, what size backpack you need - the best backpack size depends on your trip. So these are some questions that you should always ask yourself when shopping for a travel backpack: 

Consider Your Trip Length

Obviously, a weekend trip isn’t going to require as many clothes or as much stuff as multi-week travels. If you’re traveling for more than a few days, you’ll probably want to bring a few more clothes with you, a few more toiletries, and therefore, a larger bag. And for shorter trips, you don’t want to haul around an unnecessarily large backpack with empty space.

"For the last few years, I have been searching for a bag that has the space and durability to travel with while also having the utility to use every day. This is that bag. Series 2 Medium Knack Bag has enough space to hold 3-4 days of clothing and the laptop pocket is awesome. It is highly functional and exactly what I was looking for." - Neo F

MORE >> Watch this video: 13 Days in Europe With Just One Bag

Review Where You Are Traveling

If you’re traveling somewhere warm and sunny, your lighter clothes will take up less space and you can probably get by with a smaller bag. But the clothes that you bring to colder winter climates are bulky, so you’ll need a bigger backpack. Similarly, if you’re traveling for business, you’ll need room for your professional clothes and technology (hint- make sure that your laptop fits). Consider all the ‘extras’ that you’ll need for wherever you’re going - cameras, toiletries, wool socks, etcetera. Even if you’re going on a shorter trip, you may need to bring a larger bag to fit everything that you need.

MORE >> How to have a one-bag business trip

Determine Your Packing Style

Some people are minimalist packers - meaning they can get by anywhere with a couple of t-shirts and a cell phone. Others are just-in-case packers and only feel comfortable traveling with backups for everything. There’s no right way to pack - but you need to be realistic about what your packing style is. If you’re a just-in-case packer, you should consider getting a larger travel backpack with plenty of room, smart organizational features, and pockets for all your items.

When choosing your backpack, keep long distances, transit, and one bag travel in mind. Look for material that’s durable, design that balances style and function, and features that make easy packing and access to your essentials simple. A thoughtfully packed backpack fits in the overhead bin, gives you room for all your items, and makes every trip—from commutes to long journeys—more effortless.

And whatever your packing style is, you can always benefit from making a packing list!

MORE >> How to make the perfect packing list

What size backpack can you carry on a plane?

The Knack team always prefers to travel with a carry-on only - it lets us keep everything with us on the plane, skip lines, and save money on checked bags. Your backpack will generally be allowed as a carry-on if it’s under 50 liters, or about 22 x 14 x 9 inches. Get a 45-liter backpack if you want to carry as much stuff as possible on the plane with you. 

All Knack Packs are carry-on friendly backpacks (under 50 liters).

MORE >> Compare all Knack sizes and styles here

A backpack can be used as a personal item on most jets - meaning that it fits under the seat in front of you - if it’s under 20 x 12 x 9 inches. Like carry-on rules, some airlines have a larger personal item allowance and some are much more restrictive, so it is always important to check your airline’s website beforehand if you want to ensure your backpack counts as a personal item. Small and Medium Knack Packs will fit under the seat on most jets, even when expanded!

Carry-on Backpack Quick Tips

• Maximum dimensions: The Knack travel backpack fits within most airline carry-on backpack limits at 22 x 14 x 9 inches, easily sliding into the overhead bin. Unlike some bulkier packs from Patagonia or Osprey, Knack keeps weight and material in mind so you can pack clothing and items for long distances without stress.
• Ideal capacity: At 40–45 liters, Knack offers a spacious main compartment with smart organizational features and plenty of pockets, balancing easy packing with everyday work essentials. Many competitors sacrifice room or access in this size.
  Larger backpacks: Packs over 45–50 liters, like some Cotopaxi or Peak Design models, may exceed carry-on limits and require checking, while Knack stays carry-on friendly for one bag travel.
• Personal item option: Smaller Knack backpacks can double as a personal item, keeping a second bag or thing handy on transit. Thoughtful shoulder straps and hip belt design make it comfortable for frequent travelers, combining style and durability that lasts a year or more.
  
  

Travel tip: Look for a backpack with a dedicated laptop compartment, spacious main compartment, and organized pockets to carry your essentials comfortably while staying within the size limit.

Travel backpacks vs. outdoor backpacks

Outdoor backpacks - like those for hiking - are usually high-capacity and start at about 45 liters. While they might have lots of space, they aren’t great for general non-camping travel. Organizing clothes, tech, and toiletries in the deep top-loading compartments intended for camping gear is a nightmare as everything gets jostled around and the little things fall to the bottom of the bag. Outdoor backpacks make it extremely difficult to access what you need when you need it. And not to mention - the dangling gear straps and zippers can get caught in the baggage carousel. 

A general-purpose travel backpack will have more pockets for organization and will look more professional when you’re traveling around cities and checking into hotels. 

I have taken my Knack Pack on over 20 trips. I arrive at the hotel, empty everything in a drawer, compress the bag, and it's the perfect computer bag. Caught in a downpour and everything stayed dry inside. Expanded, it fits perfectly under the seat on every flight." - Raul A

Consider an expandable backpack for travel

An expandable backpack might just be the best size backpack for travel, as it has the flexibility to add more space when you need it, and it can be used for multiple purposes.

The best expandable backpack size for overnight trips

A 20-30 liter backpack is great for an overnight trip. The Small Knack Pack is only 15 liters when unexpanded - perfect for carrying your essentials with you during the day -  but the expandable travel compartment allows you to easily pack a day’s worth of clothing.

The best expandable backpack size for trips of a couple nights

The Medium Knack Pack is 27 liters when unexpanded, and 39 liters when expanded. You can pack at least 2-3 days worth of clothes in the travel compartment, and fit a 15” laptop in the separate, sling-bottom laptop pocket. If you’re traveling for a few days and need to do some work online along the way, the Medium Knack Pack is the perfect size backpack.

Is a 40L backpack big enough for traveling?

A 40 liter travel backpack is definitely big enough for traveling, as long as you're not an over-packer. It's a good idea not to go for an overly large backpack - overpacking can be as troublesome as under-packing. After all, you don't want to lug a huge bag everywhere during your trip.

The best expandable backpack size for long trips

Unexpanded, the Large Knack Pack is a perfect 30-liter daypack. But it’s a whopping 46 liters when expanded, so you can really fit a lot of clothes and other travel essentials that you will need for longer trips. 

“I was able to make it through 3 weeks in Europe with this bag (and packing cubes) as well as use it for my work” - Jon C. about his Series 1 Large Expandable Knack Pack.

Still not sure what size backpack you need for travel? Check out this useful chart to compare all Knack Pack sizes. 

The best part about bringing an expandable backpack for travel is that it can be used as a daypack, which you’ll inevitably need when exploring your destination. You get to avoid carrying around a large clunky bag, and you save space by not having to pack a daypack inside of your travel bag. 

Travel backpack size guide

How will you use your expandable backpack? We love to hear how you plan to use - or have used - your Knack Pack for work, travel, and everyday life. So share your stories and adventures with us on social media: tag @KnackBags and use the #KnackBags when you take your Knack on journeys.