What Size Backpack Do I Need? 10 to 35 Liters by Use

For a workday with a laptop, 15 to 20 liters is usually enough. For school or a long day with a lunch box and a sweater, go for 20 to 25 liters, and for flying with just a backpack or a weekend away, 30 to 35 liters. Under 15 liters you have a small everyday bag; over 40 liters it's a pack for long hikes.

Volume is the first number on every backpack page, but it's hard to picture what 20 liters actually holds. So we went through the backpacks we have in stock right now, 85 models, and sorted them by use. Below is a summary table, then each size range in more detail.

Backpack size by use

Most of the models in stock sit in the middle: 39 of the 85 are between 16 and 25 liters. That's no accident. It's the size of a normal day with a laptop.

Small backpacks: when 10 to 15 liters is enough

A small backpack suits anyone heading out light: phone, wallet, a water bottle, maybe a tablet. The upside is weight and size. The RoamTrek Edge at 9 liters weighs 310 grams, and The Lopez at 12 liters weighs 400 grams and still has a 14 inch laptop sleeve.

You'll also find slim bags for big laptops at this size. The Intima Pro, for example, is 10 liters and 10 cm deep with a 15.6 inch laptop compartment. There's room for a laptop and papers, but not for a sweater and lunch. If those are part of your day, go one size up.

15 to 20 liters: the workday size

This is the size that works for most people commuting with a laptop. Most bags in this range have a padded sleeve for laptops up to 16 inches, like the FlexVoy Edge and the Tritonz Ultra at 16 liters. There's room left for a charger and a bottle, and the bag still doesn't stand out on the bus or in the office.

Torn between two sizes? Think about your busiest day of the week, not a normal one. If you hit the gym after work once a week, you'll feel the difference between 16 and 20 liters. And if you're after a slimmer-cut work bag, we wrote about choosing a women's backpack for work.

20 to 25 liters: school and long days

If you're out of the house all day, as students often are, you need room for one more layer: food, a sweater, a book or gym clothes. In this range you'll find the ActiveMax 2.0 Edge and the Beyond Prime at 20 liters, the Blanco Pro at 22 liters and the Hoot Pro at 25 liters, all with a 16 inch laptop sleeve.

There are expandable bags too, handy if your days vary. The Catalyst Supreme at 20 liters opens with an expansion zipper when you need more room, and stays slim when you don't.

30 to 35 liters: flights and weekends

This is where a backpack replaces a carry-on. 35 liters holds clothes for a few days along with your laptop. We have 18 bags in stock between 31 and 40 liters, like the Everest and the Orogeny at 35 liters. Before a flight, check the dimensions as well as the liters, because airlines set their limits in centimeters. We cover that in our flight backpack size guide, and all the models that suit flying are on the travel backpacks page.

Want to fit more into the same volume? Vacuum backpacks shrink the space your clothes take up, and for a weekend bag instead of a backpack we also have a weekend bag guide.

40 liters and up: packs for the outdoors

Above 40 liters, bags are built to carry weight for hours. The XL Mountaineer, for example, has a chest strap and a hip belt that moves most of the weight off your shoulders and onto your hips. The Nevo Extreme comes in 40, 45, 50 and 60 liters, the Kryos Ultra in 70 and the Slayer in 80. For work or school that's simply too big: the bag will look empty and sway on your back.

Liters don't tell the whole story: 3 things to check

• Laptop size. A 20 liter bag with a 14 inch sleeve won't take a 16 inch laptop. We've grouped backpacks for 14 inch laptops and backpacks for 15.6 to 16 inch laptops on separate pages.
• Depth. Two 15 liter bags can be very different: one slim and tall, the other deep and short. Depth is listed in the dimensions on the product page.
• The weight of the bag itself. Around 20 liters you'll find bags from 670 grams to 1.2 kg. If you walk a lot, you'll feel it by the end of the day.

Still not sure? Our guide to choosing a laptop backpack also covers compartments and straps. And you'll find the full range, from small daypacks to trekking packs, on our backpacks page.