Guides & Advice
Backpack Sizes Explained: How Many Liters Do You Really Need
Jul
Backpack sizes are measured in liters, the volume the main compartment and pockets can hold. As a rule of thumb, under 20 liters is a light daypack, 20 to 30 liters covers a workday or a school day, 30 to 45 liters is the range for a weekend or a short trip, and anything above 45 liters belongs to long stays and trekking. The number on the label is only half the answer, though: past 30 liters, the way the backpack carries its load matters more than the extra space. If you already know you need a big one, our large backpacks for men start at that threshold.
Backpack sizes chart: liters, use and typical load
The chart below is the reference our French buying guide on large backpacks is built on, extended to the smaller sizes. The typical load is what a backpack of that size weighs once it is actually filled for the use in the second column, and it tells you more than the volume alone. Small sizes suit a foldable backpack that you carry as a spare; the upper rows are where structure becomes essential.
| Size | What it holds | Typical load when full | Carrying system needed |
|---|---|---|---|
| 10 to 20 L | Water bottle, jacket, wallet, a tablet | 2 to 4 kg | Simple straps |
| 20 to 25 L | Laptop, charger, lunch, a book | 4 to 6 kg | Padded straps |
| 25 to 30 L | Office kit plus gym clothes | 5 to 7 kg | Standard straps are enough |
| 30 to 38 L | Clothes for a two day weekend | 7 to 10 kg | Wide straps and a chest strap |
| 38 to 45 L | Three to four days of travel | 10 to 14 kg | Structured back panel |
| Above 45 L | Long trips, camping gear | 14 kg and more | Hip belt, a different category |
What a liter means on a backpack
A liter is 1,000 cubic centimeters. Multiply the height, width and depth of a backpack in centimeters, divide by 1,000, and you get a rough volume: a bag of 45 x 30 x 20 cm holds about 27 liters. The real figure is a little lower, because the shape is rounded and the padding takes space, but the calculation is a good way to check a product page that gives dimensions and no volume, or the reverse.
Two backpacks with the same number can still feel very different. A rigid laptop compartment, thick foam on the back or a separate shoe pocket all eat into the volume. Brands also count differently: some include the side pockets and the front pocket, others only the main compartment. Treat the liters on the label as a range of plus or minus a few liters, and compare the dimensions when two models are close. Our laptop backpacks for men list both on each product page.
How to choose the right size for your use
Start from what goes inside on a normal day, not from the biggest trip of the year. A backpack that is too large for daily use sags, swings when you walk and invites you to carry more than you need. Then add a margin of about 20 percent for a jacket or shopping.
- Commute and office: 20 to 25 liters for a 14 or 15.6 inch laptop, its charger and the essentials.
- Office and gym on the same day: 25 to 30 liters, ideally with a separate compartment for clothes.
- School or university: 25 to 30 liters, books take more room than a laptop.
- Weekend away: 30 to 38 liters if you pack light and wear your bulkiest clothes.
- One week with one bag: 38 to 45 liters, with an organized packing method.
For trips, the size also depends on where the backpack travels. A travel backpack that opens like a suitcase lets you use every liter, while a top loader wastes space at the bottom.
Above 30 liters, the harness decides
The word large appears on product pages from 20 liters on, which means very little. The threshold that matters is the point where the carrying system has to change. From about 30 liters, a filled backpack regularly goes past 8 kg, and above that weight simple straps are no longer enough. Four details then make the difference between a big backpack and a good big backpack, and they are the criteria we check on every model of our large backpack range:
- Strap width: 5 cm at least. Narrower straps press on the trapezius and the pain comes before the fatigue.
- A chest strap: it pulls the shoulder straps together and moves part of the load onto the rib cage.
- A structured back: a stiff panel or a light frame stops the bag from wrapping around your back.
- Access to the bottom: a side or front zip, otherwise what you packed first becomes unreachable.
Backpack sizes and airline cabin limits
Airlines do not limit liters, they limit dimensions, handles and straps included. A backpack of about 20 to 25 liters usually fits the under-seat allowance, the free small bag of most fares, as long as it stays slim. Our guide to the personal item size lists the limits airline by airline, for example 18 x 14 x 8 inches on American and Frontier. In Europe, the free easyJet bag is 45 x 36 x 20 cm, detailed in our easyJet cabin bag size guide. Depth is where most backpacks fail the sizer, so measure yours packed.
Real backpack sizes from our range
To make the chart concrete, here are backpacks from our catalogue whose volume is stated in the product name, from the smallest to the largest. Each product page gives the details you need to compare them with the chart above.
- See the 15 L foldable backpack: a spare bag that folds into its own pocket, for a city day.
- See the 18 L hiking backpack: a daypack for a half day walk with water and a layer.
- See the 30 L travel backpack: the size where a weekend starts.
- See the 40 L outdoor backpack: three to four days, structure required.
- See the 60 L tactical backpack: long trips and bulky gear.
In short
Pick the backpack size from your everyday load, then add a fifth for margin: 20 to 25 liters for work, 25 to 30 for work and gym, 30 to 38 for a weekend and 38 to 45 for a week. Past 30 liters, check the straps, the chest strap and the back panel before the volume. To compare models that pass that test, browse our large backpacks for men.
Frequently asked questions
What size backpack do I need for everyday use?
For a laptop, a charger and the essentials, 20 to 25 liters is enough. Go up to 30 liters if you also carry gym clothes or books every day.
Is a 30 liter backpack big?
It is the upper end of everyday sizes and the lower end of travel sizes. A 30 liter backpack holds a weekend of clothes if you pack light, and it is often the largest size that still fits under an airline seat.
How many liters do I need for a week of travel?
Between 38 and 45 liters with a structured back and organized packing. Below that, you need to wear your bulkiest clothes and plan on doing laundry.
How do I convert backpack dimensions into liters?
Multiply height, width and depth in centimeters and divide by 1,000. A backpack of 45 x 30 x 20 cm gives about 27 liters, slightly less once you account for the rounded shape and the padding.
Does a bigger backpack hurt your back?
Volume itself does not, weight does. A large backpack filled to 12 kg on thin straps will hurt, while the same load on wide straps with a chest strap and a stiff back panel stays comfortable.
