Backpack Size Guide: Everything You Need To Know

Want to get a backpack but you aren’t sure how to pick the right size for you? Your troubles have come to an end – this backpack size guide will tell you everything you need to know about backpack sizes!

We’ll guide you through everything, from choosing the right size based on your body shape to calculate the approximate capacity if the manufacturer failed to list the volume of the bag. We’ll also go through the average capacities of basic backpack types, just to give you an idea of what to expect.

So, if you can’t figure out which size is right for you, if you’re having trouble visualizing the size of an 80-liter backpack or if you want to know how to figure out the capacity on your own, you are definitely in the right place!

How Big Is A 20/40/60/80-Liter Backpack?

When you’re looking at different backpacks, you usually see their capacity expressed in liters, and sometimes cubic inches. But, until you’ve actually held the backpack in your hands, it can be pretty difficult to visualize just how big it actually is.

Let’s do it the easy way – I assume you have at least one small, 18-20” suitcase. The capacity of those is usually around 35-40 liters (up to 2441 cubic inches), which is pretty much the same capacity you get in a standard 22” carry on backpack.

So, a 20-liter backpack can hold as much stuff as a single compartment of the suitcase, while an 80-liter backpack could hold twice as many items as the entire suitcase.

Backpacks are a lot bigger than they seem, especially the ones that have lots of external pockets and multiple compartments. That’s another thing people often forget about – the capacity of every single pocket is included in the total capacity of the backpack.

So, if you want a bag with a massive main compartment, you should definitely look for something that doesn’t have too many outer pockets, since those bags can ultimately fit fewer items inside their main compartments.

You can see here that my Nebula (which has a 34-liter capacity) is in fact a bit bigger than my 19” carry on. It kind of looks smaller, but only because the pack is flattened – once you get some air inside those compartments, it’s actually wider than the luggage.

Their capacities are nearly identical – I think the Nebula could even fit more things inside, simply because it has that stretchy front pocket while the suitcase is pretty rigid. But we normally think of backpacks to be the smaller alternatives – perception can be quite deceptive sometimes, don’t you agree?

Which Size Is Right For You?

I always tell people to consider two things when choosing a backpack size – how much stuff do you plan to put in it and what’s your body shape.

The best backpack for you is the one that ticks all your boxes and that has plenty of space for the items you consider necessary, whether we’re talking about laptop or hiking packs. And it’s better to get something that has more capacity than you need than to have a backpack that can’t fit all your must-haves.

When talking in general terms, 20-liter backpacks are usually okay for everyday carry, if you don’t normally have too many items in your bag. Backpacks that have a capacity of 30-50 liters are good for longer adventures and even shorter trips, as long as you’re not putting too many bulky items inside them.

If you are carrying bulky gear (for hiking or climbing), you will need at least a 50-liter bag, even for just overnight expeditions. Anything above 50 and 60 liters is meant for backpackers and travellers who embark upon pretty long adventures.

Backpacks that are over 120 liters can fit as much stuff as 32″ checked suitcases, which can usually get you through a two-week vacation.

The second thing you need to consider is the actual size of the pack vs. the size of your torso. If you aren’t able to try it on in a store, definitely measure how the pack dimensions compare against your torso. If it hangs too low, go with something smaller because that will ultimately be more comfortable.

The waist belt should be slightly above your hips for the most comfortable fit – if it’s too low, it’s not going to fit well, meaning that it’s not going to do much in terms of actually transferring the weight onto your hips. And the sternum strap should be above your chest – getting this to fit well can sometimes be challenging for the ladies, especially if a particular backpack doesn’t have a height-adjustable sternum strap.

Of course, this only applies to smaller backpacks that have a fixed harness. The massive, 80 liter+ bags normally have height and width adjustable harness, so it’s pretty easy to make those fit you well.

And, keep in mind that you’re probably going to adjust the backpack as you’re moving around – what felt incredibly comfortable at home might not be so comfy after three long hours on the trail.

Average Sizes Of Different Types Of Backpacks

Backpacks come in many shapes and sizes, and it almost exclusively depends on the type of bag. That’s why I always tell people that they should always know exactly what type of backpack they want, so they know which sizes to look for. From there, it’s simply a matter of selecting a backpack that has all the features you need.

Hydration backpacks are normally the smallest ones, with an average capacity of 7-10 liters. However, you can find hydration backpacks with 3-liter capacities, and those are designed to only fit the reservoir and a couple of necessities. And you can also find hydration packs that have a capacity of 15-20 liters, but those are much closer to small hiking backpacks.

Everyday packs have an average capacity of anywhere between 15-35 liters. Their capacity heavily depends on the number of compartments and external pockets, and it’s very easy to find something on either side of the range.

It’s the same with laptop backpacks – most manufacturers try to make them as big as possible with a plethora of different compartments and organizer panels so that you get enough space for all those smaller accessories.

Hiking backpacks are probably the most confusing out of the bunch, simply because there’s so many different options and their capacities can range anywhere from 20 to 60 liters. And that’s because there are so many different varieties – lightweight daypacks, multi-day expedition backpacks, tactical backpacks, etc.

In general, anything up to 30-35 liters should be more than enough for an easy day hike. But if you’re going on a more demanding hike that requires you to carry crampons, ropes, hiking poles, and all the other gear, then 50-60 liter packs are much more appropriate.

With travel backpacks, there’s even more variety. Carry on travel backpacks normally have a max capacity of 40 liters, which is equivalent to the capacity of a standard carry on suitcase.

This is because they are usually designed to fit TSA carry-on restrictions, and the majority of them are maxed out in every dimension. This is ideally some 45 linear inches, and the reason most standard backpack sizes are 22” x 14” x 9”, give or take an inch here and there.

But when you look at travel backpacks that are supposed to be checked, all rules go out the window. And the same applies to backpacking packs – their capacities range from 60 – 150+ liters. Generally, when we get to the bigger bags that are designed for long camping trips, climbing expeditions, and travel, average capacity stops being a thing.

With these backpacks, you normally get three size options of a single version. The smallest option is usually about 50-70 liters, the medium one is anywhere between 70-100, and the largest packs are generally 100+ liters.

However, this heavily depends on the brand you are looking at, the exact type of backpack, and whether or not it’s gender-specific. Women’s backpacks tend to be smaller than men’s backpacks because they are designed to be more narrow, in order to fit female torsos better. It’s not a massive difference and it’s barely even noticeable on the smaller bags.

Calculating The Capacity Of A Backpack

Sometimes manufacturers won’t give you any information about the capacity of a bag, which sucks. You’ll either have to shop blind or try to find any info about the total volume of a backpack – I find that it often pays off to check the reviews on Amazon, or ask the seller yourself if you can’t find the answer.

More often than not the exact volume of the pack will be specified on the tags, so somebody will be able to give you an answer.

You could also try calculating the volume of a bag yourself – it’s surprisingly simple, but you can only get an approximate volume. I’m not going into details here because that would entail a lengthy maths lesson, but the basic idea is that a volume of a shape is calculated by multiplying the dimensions of its length, width, and height.

The main issue here is that you can only get a perfect result when you are looking at a smooth box. Backpacks are normally more oval-shaped at the top and square-shaped at the bottom, so you would actually have to do fractions and what not to get the closest result. Since I doubt anybody actually has the time or energy to do that, a basic online volume calculator will have to do.

The best thing about this online calculator is that you just input the numbers, select the exact length/volume unit, and all the work is done for you. You can calculate volume from millimeters to cubic inches or from inches to liters, and it’s ridiculously easy.

If you’re looking at a backpack that’s more oval-shaped at the top, you will get a result that’s higher than its actual volume. I tried calculating the capacity of Osprey’s Daylite Plus – the calculator said it was 28 liters, but in reality, the capacity of the bag is actually 20 liters. The bigger the backpack, the higher the discrepancy is going to be.

If you’re looking at a backpack that’s square or rectangular, chances are you are going to get pretty accurate results. However, keep in mind that you are only calculating the capacity of the main compartment – the online calculators can’t account for the different external pockets a bag might have, which serve to increase its total capacity.

I tried this out with the Fjallraven Kanken Classic, since it’s as close to a clean rectangular as you can get. The backpack has a total capacity of 16 liters, while the multiplication of its dimensions resulted in an estimated capacity of 13.3 liters – that’s because the front pocket of the backpack isn’t accounted for in those dimensions, so it’s a fairly accurate result.

And that would be it for this backpack size guide. I hope I covered everything you were curious about – if not, just let me know and I’ll be sure to go back and edit the post!