What Size Backpack for Hostels and a Budget Airline

For a trip built around budget airlines, the right backpack holds roughly 20 to 30 liters. The airline's sizer at the gate sets that limit, and the length of the trip does not. Frontier's free personal item may measure no more than 14 by 18 by 8 inches, "including handles, wheels, and straps," according to Frontier's own baggage FAQ. Multiplied out, that box holds about 33 liters. A packed bag with rounded corners and padded straps fills less of it. Anything bigger becomes a paid carry-on, and the fee is charged on every flight.

The usual advice scales liters to days, and for this kind of trip that gets the order wrong. The airline fixes the ceiling. The trip decides only how close to that ceiling the bag should go. JetSetEquip, a gear site, puts the sweet spot for "budget/extended travel" at 20 to 30 liters, so a month on the road does not by itself justify a bigger bag than a week does.

Bag makers rate capacity in liters. Airlines publish outside dimensions. The two figures are only loosely related, and the gate checks only the second.

Ryanair's free bag shows the gap. The limit is 40 by 30 by 20 cm, which is a 24-liter box. Which? Travel calculated its usable volume at about 20 liters, and Wizz Air's bag of the same outside size at about 24. easyJet's free under-seat bag, 45 by 36 by 20 cm, comes to about 32.4 liters.

Of the three dimensions, depth is the one that bites. PackFitter calls Ryanair's 20 cm depth (7.9 inches) "the defining feature" of the policy, because the same limit applies to both the free bag and the paid one. On Frontier, PackFitter names the 8-inch depth as the most common reason a bag fails. Backpack Brands puts the general point plainly: "A bag can be 45 liters but shaped like a cube, making it too wide for a sizer. Shape matters more than volume."

PackFitter's Ryanair table sets out how liter ratings tend to fare against those limits:

• About 25 L: commonly fits as the free item.
• About 30 L: borderline, depending on shape and how deep it packs.
• About 35 L: fits the paid cabin tier only if it stays at 20 cm deep or less.
• About 40 L: "risky."
• 45 L and up: "too large."

Frontier. The free item is 14 by 18 by 8 inches. The paid carry-on can be 24 by 16 by 10 inches and up to 35 lbs, a box of roughly 63 liters. Frontier says baggage costs least when bought at booking and more at the airport. A 40-liter pack fits that carry-on easily, but it rides as a paid bag on every leg.

Ryanair. The free bag is 40 by 30 by 20 cm. A 55 by 40 by 20 cm cabin bag requires the Priority & 2 Cabin Bags add-on. Which? priced it at £12 to £36 booked in advance and £20 to £60 added later. TripProf put the gate fee for an oversized or extra bag at roughly €70 to €75. It also noted that Priority is capped at about half the cabin and can sell out.

easyJet. The free under-seat bag is the most generous of the three at 45 by 36 by 20 cm and 15 kg. The large cabin bag is advertised "from £5.99," but Which? found it typically runs about £23 or more.

The products show the same pattern. JetSetEquip's 2026 roundup lists a Vancropak "40L" pack at 18 by 14 by 6.5 inches for $45 to $55, and an Amazon Basics 40L, expandable by 10%, for $40 to $50. The same roundup covers MATEIN's 40 to 45 liter models but gives no dimensions or price for them. The Vancropak's listed dimensions describe a box of about 27 liters. That box passes Frontier's free sizer on paper and clears Ryanair's 20 cm depth limit. A bag sold as 40 liters that measures like 27 is a better buy than its label suggests.

The strongest objection is that a month of hostels, with a towel, a lock and some cold-weather layers, needs more room than an under-seat box allows. Booked early, the room is cheap. A Ryanair cabin bag at £12 costs less than a meal at the airport.

This is fair, and for a single round trip it can win. Two things go against it. The fee applies per flight: four Ryanair legs at £12 to £36 each come to £48 to £144, which is more than the price of the bag. And a larger allowance still has to clear the depth limit. Ryanair's paid tier keeps the 20 cm depth, which is why PackFitter rates most 40-liter packs as risky even when the fee has been paid. At 50 liters and above, the bag is in PackFitter's "too large" class and likely to be checked.

The sources here say nothing about hostel stairs, lockers or night buses, so the case against a big pack rests on the airline arithmetic alone. That case is strong enough. How strictly bags are measured at the gate varies by carrier, but the fee schedules assume that they are.

In practice, this means four checks. Read the listed dimensions rather than the liters. Compare them with the smallest free allowance on the itinerary. Measure the depth when the bag is packed, not when it is empty. If a paid bag is unavoidable, buy it at booking, because every carrier above charges more later.

The rules will loosen, but not yet. The EU's new passenger rules were published in the Official Journal on 2 October 2026, The Portugal News reported. They guarantee a free personal item plus a free cabin bag of up to 7 kg, with length, width and height adding up to no more than 100 cm. The rules are expected to apply from late 2027, with 23 October 2027 cited as the start date. Until then, current fees stand. The guaranteed personal item, 40 by 30 by 15 cm, is shallower than Ryanair's present 20 cm, and Lufthansa Group's Basic fare already limits its free item to that size.

The bag to buy is one that fits the shallowest box on the whole itinerary. The trip is then packed to fit the bag.