What Size Backpack Fits Under an Airplane Seat?
Key takeaways
- A backpack around 17 x 13 x 8 inches fits under the seat on nearly every US airline, including United's strict 17 x 10 x 9-inch personal-item limit.
- United is the strictest major US carrier at 17 x 10 x 9 inches. American allows 18 x 14 x 8. Delta and Alaska set no exact size, only "must fit under the seat."
- Soft-sided backpacks in roughly the 20- to 30-liter range usually qualify as personal items. Measure the bag fully packed, since handles and straps count.
Passengers queue at Taichung Airport, where as at most airports each traveler boards with one carry-on plus one personal item. Photograph: Saimmx/Wikimedia Commons
Image Credits: Wikimedia Commons
A "personal item" is the second bag airlines let you bring into the cabin for free. It is the smaller one: a purse, laptop bag, briefcase, or backpack that has to fit under the seat in front of you, not in the overhead bin.
The tricky part is that every airline defines "fits under the seat" differently. This article explains the standard personal-item size for a backpack under an airplane seat. It covers personal-item dimensions by airline, how those numbers translate to backpack liters, how to measure your own bag, and tips for choosing one that fits.
What is a personal item?
Most airlines allow two free cabin bags: one full-size carry-on for the overhead bin, and one personal item that goes under the seat in front of you. The personal item is where your in-flight essentials live.
Backpacks qualify as personal items on essentially every airline, as long as they meet the size limit. The Points Guy's baggage guide notes that American explicitly lists backpacks alongside purses, briefcases, and laptop bags as examples of personal items.
A backpack beats a rigid case as a personal item for one simple reason: soft fabric compresses. Trecklist's 2026 personal-item guide points out that soft-sided bags are more forgiving than hard-shell ones when the under-seat space is tight. The published dimensions are a guide; the real rule is whether the bag slides under the seat.
Personal item dimensions by airline
There is no universal standard. Each airline publishes its own maximum, and some publish no number at all. The tables below combine the airlines' published rules as compiled by Trecklist's 2026 personal-item guide and Skyscanner's cabin-baggage guide, cross-checked against travel-press reporting.
US airlines (inches)
| Airline | Max personal-item size | Weight rule |
|---|---|---|
| United | 17 x 10 x 9 | None published |
| American | 18 x 14 x 8 | None published |
| Southwest | 18.5 x 13.5 x 8.5 | None published |
| JetBlue | 17 x 13 x 8 | None published |
| Frontier | 18 x 14 x 8 | 35 lb combined with carry-on |
| Breeze | 17 x 13 x 8 | None published |
| Delta | No stated size; must fit under the seat | None published |
| Alaska | No stated size; must fit under the seat | None published |
United's 17 x 10 x 9-inch limit is the strictest of any major US carrier. That makes it the one to design around if you fly multiple airlines. American's 18 x 14 x 8-inch rule is confirmed across several sources. The Points Guy quotes it directly, and luggage maker CabinZero repeats the airline's published policy word for word.
Delta and Alaska take a different approach and publish no exact dimensions. Their only stated test is whether the bag fits under the seat in front of you, according to Trecklist's compilation of the airlines' published rules.
Frontier stands out in two ways. Its personal-item limit is a familiar 18 x 14 x 8 inches, but it also enforces a 35-pound combined weight limit for the personal item plus the carry-on, per Skyscanner's guide. US weight limits are otherwise almost never enforced on personal items.
One more US note: guides' figures for Allegiant disagree with each other. Skyscanner lists 18 x 14 x 8 inches and Trecklist lists 7 x 15 x 16 inches. Check the airline's site before flying Allegiant.
European budget airlines (centimeters)
Budget carriers such as easyJet and Ryanair enforce small-bag limits more strictly than most US airlines, often with baggage sizers at the gate. Photograph: Infrogmation/Wikimedia Commons
Image Credits: Wikimedia Commons
| Airline | Max small-bag size | Weight rule |
|---|---|---|
| easyJet | 45 x 36 x 20, including handles and wheels | Up to 15 kg, must be liftable |
| Ryanair | 40 x 20 x 25 | 10 kg |
| Wizz Air | 40 x 30 x 20 | Check the airline's site |
European budget carriers are noticeably stricter, and they enforce their limits with baggage sizers at the gate. The Glasgow Times' 2026 comparison, which quotes the airlines' own websites, puts easyJet at 45 x 36 x 20 cm including handles and wheels, with a 15-kg cap.
Ryanair's own site lists its free small bag at 40 x 20 x 25 cm, according to a Mirror comparison of the airline's published rules. Other guides render the same limit as 40 x 30 x 20 cm, so treat 40 cm on the longest side as the safe reading. Ryanair also keeps a 10-kg limit on the bag, as reported by Lanzarote Retreats' baggage guide.
HolidayExpert went further and physically tested 20-, 30- and 40-liter bags against the Ryanair, easyJet and Wizz Air sizers at an airport in 2026. The tests confirmed in practice that published sizer dimensions are what actually get enforced at the gate.
How those dimensions translate to backpack liters
Airlines publish dimensions, not liters, so backpack shopping requires a rough conversion. A concrete data point helps: an Amazon-listed underseat roller measuring 14 x 15 x 8 inches lists 23.85 liters of capacity in its specifications.
Scale that up to the common personal-item footprint of 17 x 13 x 8 inches and you land around 25 liters. So as a rough guide, a backpack in the 20- to 30-liter range is the sweet spot for under-seat travel on strict airlines like United or Ryanair.
Go much above 30 liters and the bag starts to look like an overhead-bin bag. Trecklist's guide warns that most 30-liter-plus hiking backpacks are too tall for personal-item status. A 40-liter travel backpack almost always has to go overhead.
In metric terms, the widely cited safe size of 17 x 13 x 8 inches converts to about 43 x 33 x 20 cm. If you are shopping from a European brand, look for a "cabin small" or "underseat" backpack near those numbers.
How to measure your backpack
Measure the bag packed, not empty. A half-full backpack always measures smaller than the one you will actually carry.
Include everything the airline would include: handles, straps, and any external pockets. Several guides stress that wheels and handles count toward the published dimensions. The same logic applies to chunky top handles and stuffed front pockets.
Compress it the way the gate agent will see it. Cinch compression straps, tuck loose webbing, and push the bag into the rough shape it takes when full. Then compare the result to the strictest airline on your itinerary, not the most generous one.
Expect enforcement to stay human. Travel Tomorrow reported in October 2025 that American Airlines removed its physical bag sizers. Staff now "use their judgment," erring "on the side of the customer" and checking only bags that are "clearly oversized." That is a trend, not a guarantee. Other airlines and other gates still use sizers.
Frequently asked questions
Can I bring a backpack as my personal item instead of a carry-on?
Yes. Every major airline lists backpacks as acceptable personal items. You can also bring a full-size carry-on on top of it on most tickets. On some basic-economy and budget fares, the personal item is the only free bag.
What happens if my backpack is slightly bigger than the limit?
It depends on the airline and the gate agent. Soft backpacks that are slightly over often slide under the seat anyway. But a gate agent can require you to check an obviously oversized bag, sometimes for a fee.
Do budget airlines like Ryanair and easyJet have different rules?
Yes, and they are stricter. Ryanair's free small bag is about 40 cm on its longest side and easyJet's is 45 x 36 x 20 cm. Both are well below the US big-carrier limits, and both airlines enforce them with sizers.
Is there a weight limit for personal items?
Usually no. US carriers publish no personal-item weight limits at all. Frontier is the exception with a 35-pound combined limit for the personal item plus carry-on. In Europe, easyJet caps its small cabin bag at 15 kg and Ryanair at 10 kg.
The bottom line
Buy for the strictest airline you fly, not the most generous one. A soft-sided backpack around 17 x 13 x 8 inches, roughly 25 liters, clears United's tight limit and fits under the seat on nearly every other airline too.
Before you travel, measure your bag fully packed and compare it to the smallest number in the tables above. Two minutes with a tape measure beats a surprise gate fee every time.
Tags: backpacks, carry-on rules, airline baggage, packing, air travel
Rive News is the research desk behind Rive News's explainers, covering travel, food, fitness, parenting and pets with sourced, plain-English guides.
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