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. The best carry-on travel backpack size 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. 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.