Five liters sounds like nothing. It is the volume of a large water bottle, a pair of shoes, maybe a sweater. Yet the gap between a 40L and a 45L travel backpack is one of the most consequential gear decisions a traveler makes, because those five liters sit exactly where airline carry-on rules get enforced. Choose wrong and you pay checked-bag fees on every flight. Choose right and you never think about it again. Here is how to decide.

What the numbers actually mean

Backpack capacity is measured in liters of internal volume, but two bags with the same rating can pack very differently. Shape matters: a boxy 40L bag with a clamshell opening often swallows more usable gear than a tapered 45L hiking pack. Internal organization eats volume too. A bag stuffed with dividers and pockets may carry less clothing than a simpler bag of the same size.

As a rough guide, travelers consistently find that 40L holds about a week of clothes with laundry, a pair of spare shoes, toiletries, and a laptop kit. A 45L bag adds roughly one more outfit, a bulkier jacket, or a second pair of shoes. That extra room sounds trivial until you are packing for a cold destination or a trip longer than ten days.

The airline carry-on reality

This is where the decision gets real. Most major international carriers allow a cabin bag around 55 × 40 × 23 cm, which accommodates most 40L to 45L bags comfortably. The problem is the strict end of the market. Ryanair's larger cabin bag allowance, easyJet, Wizz Air, and similar budget carriers often enforce 55 × 40 × 20 cm, and some weigh cabin bags at the gate with limits of 7 to 10 kg.

A well-designed 40L travel backpack typically measures within a whisker of the strict limits and compresses to fit sizers. A fully packed 45L bag usually does not. Experienced carry-on travelers report the pattern repeatedly: the 45L bag sails through on legacy airlines, then gets gate-checked for €40 on the budget flight that connects the trip. Travelers flying mostly full-service carriers face little risk either way. Travelers mixing in budget airlines, which is most people traveling long-term, feel the difference constantly.

Weight is the quieter issue. An empty 45L bag plus 45 liters of gear often lands at 11 to 13 kg, over the weigh-in limit for many airlines even when the dimensions pass. A disciplined 40L load is easier to keep under 10 kg.

Packing capacity, side by side

Here is what each size realistically holds, based on the packing lists travelers actually use:

  • 40L: 5 to 6 tops, 2 bottoms, 5 days of underwear and socks, 1 packable jacket, 1 spare pair of shoes, full toiletries, laptop and chargers, with a little breathing room
  • 45L: everything above, plus a warmer layer or sweater, a third bottom, and either a second spare pair of shoes or a small daypack folded flat

The honest truth most long-term travelers land on: both sizes require laundry every five to seven days. Once laundry is part of the system, the 40L covers trips of any length in moderate climates. The 45L earns its keep in two specific situations: cold-weather travel where layers are bulky, and trips where you carry extra gear like camera equipment, a full work setup, or climbing shoes.

Which trips suit each size

Choose 40L when:

  • You fly budget airlines regularly, especially in Europe or Southeast Asia
  • Your trips are mostly warm or moderate climates
  • You want to commit fully to carry-on only travel
  • You value a lighter, more compact bag on transit days

Choose 45L when:

  • You fly mostly full-service airlines with generous cabin allowances
  • You travel to cold destinations where a down jacket and layers are non-negotiable
  • You carry a serious work or photography kit alongside your clothes
  • You accept occasional gate-check fees as the cost of extra space

There is also a middle path many nomads swear by: a 40L bag with an expandable or compressible design, kept cinched tight for strict flights and loosened when rules allow. It is a compromise, but a clever one.

A real-world packing test

To make the difference concrete, travelers who have packed both sizes describe the same exercise. Take a standard one-week list: five tops, two bottoms, five sets of underwear and socks, a rain shell, one spare pair of shoes, toiletries, and a laptop kit. In a 40L bag, that list fills roughly 85 percent of the space with compression cubes in use, leaving just enough slack for souvenirs. In a 45L bag, the same list fills around 70 percent, which invites the dangerous thought: what else fits? The answer, usually a third pair of shoes or a second jacket, is exactly how carry-on bags become checked bags.

The test flips for winter travel. Swap the rain shell for a down jacket, add a sweater, thermal base layers, and bulkier socks, and the 40L bag is suddenly straining at the zippers while the 45L still closes comfortably. Climate, more than trip length, is what separates the two sizes in practice.

What about 35L or 50L?

The size ladder does not stop at these two options, and the neighbors are worth a quick mention. A 35L bag suits committed minimalists, weekend travelers, and warm-climate nomads who do laundry constantly; it is the safest possible answer for strict budget airlines but demands real discipline. A 50L bag crosses a line: nearly all airlines treat it as a checked bag, so it suits travelers who have accepted checking luggage and simply prefer a backpack format for rough terrain or long overland routes. Most travelers weighing 40L against 45L are really trying to stay carry-on legal, which is why 50L rarely belongs in the same conversation.

Comfort and carry weight

Five extra liters is five extra kilograms of temptation. Travelers frequently observe that bag size determines pack weight more than discipline does: whatever space exists gets filled. A packed 45L bag on a long walk through Bangkok or Lisbon is meaningfully harder on your back than a packed 40L. If you choose the bigger size, a proper hip belt stops being optional.

Common questions about backpack size

Is 45L too big for carry-on? Not automatically, but it is risky. Many 45L bags technically meet full-service airline dimensions, yet a fully packed one often fails sizers and weight checks on budget carriers. If carry-on certainty matters, 40L is the safer bet.

How many days of travel does 40L cover? Indefinitely, once laundry is part of the plan. Clothes for five to seven days, washed weekly, covers a weekend trip or a year abroad equally well.

Do liters measure the same across brands? Roughly, but not exactly. Shape, opening style, and built-in organization change how much usable space a rating delivers, so two bags with the same liters can pack differently.

Our recommendation

For most travelers, 40L is the right answer. It forces the packing discipline that makes travel easier, fits the strictest common carry-on rules, and covers any trip length once laundry enters the routine. The 45L is the right answer for a smaller group: cold-climate travelers, gear-heavy nomads, and those loyal to full-service airlines. If you are still weighing specific models, our tested picks for the best travel backpacks break down the top bags in both sizes, and the minimalist packing list shows exactly what fits into a 40L bag. Pick the size that matches your airlines and your climate, then stop second-guessing it.