Personal Item Size Limit by Airline for Every US Carrier

A bag measuring 17 x 10 x 8 inches fits under the seat on every US airline that publishes a personal item limit in the sources available to us. The familiar 18 x 14 x 8, the size most personal item bags are sold to, clears only Frontier, American and Southwest, and too big by an inch or four for United, JetBlue and Breeze.

The gap between those two numbers sets the price of a free bag. On the full-service carriers a slightly oversized personal item is usually an inconvenience. On the ultra-low-cost carriers the personal item is the only free bag, and the fare assumes it fits.

Every figure above comes from the airline's own page except Spirit's, which now exists only as a historical record in KAYAK's report on the shutdown. The airline pages as summarized carry no check date. The two dated third-party restatements agree with the carrier: KAYAK gave Frontier's limit as 14 x 18 x 8 on May 11, 2026, and Upgraded Points gave it as 18 x 14 x 8 inches (45.72 x 35.56 x 20.32 cm) on June 10, 2026. Metric figures appear only where a source published them. Southwest and Breeze give inches alone.

Frontier's help center states the limit as 14 inches high by 18 wide by 8 deep and adds the clause that catches people: the measurement includes handles, wheels and straps. A soft backpack that measures 18 x 14 x 8 across the body but carries a padded grab handle on top is, by Frontier's definition, too big. The personal item is the only bag guaranteed free with a Frontier ticket. Everything else is priced dynamically. Upgraded Points put carry-ons at $29 to $115 as of June 2026, and SkySonar reported a $100 charge at the gate for a bag that fails the check.

American publishes the same 18 x 14 x 8 (45 x 35 x 20 cm), free in every cabin alongside a free carry-on. A bag over the limit goes into American's oversized or gate-check procedures. That costs less, because the carry-on allowance is already paid for in the fare.

Spirit used the same dimensions and enforced them with gate sizers, charging $79 or more for a non-compliant personal item. Spirit ceased operations on May 2, 2026, so the figure matters now only because so many bags were designed around it.

United's limit is 17 x 10 x 9 inches. That is an inch deeper than the 18 x 14 x 8 standard but four inches narrower in the dimension that matters. A bag built to 14 inches on its middle side does not fit, however soft it is.

JetBlue and Breeze both publish 17 x 13 x 8. Neither sets a weight limit on the personal item. JetBlue asks only that the owner be able to lift it. Breeze is a low-cost carrier, and its fare includes only the personal item. That puts Breeze's smaller limit in the same position as Frontier's larger one: it marks the edge of what the ticket price covers.

Southwest's support page allows 18.5 x 13.5 x 8.5 inches. Southwest is the only carrier here that also lets the personal item go in the overhead bin. The extra half inch is long in two directions and short in one: a bag that is exactly 14 inches on its middle side exceeds Southwest's 13.5. Southwest says bags that fail its size rules are checked at the gate.

For Allegiant, Delta, Alaska and Sun Country, none of the sources available to us publishes a personal item dimension, so this page gives none. The gap matters most for Allegiant. SkySonar describes Allegiant's free personal item as the tightest among the surviving budget carriers, without stating the size. A traveler booked on any of these four should take the figure from the airline's own baggage page before buying a bag. No comparison table substitutes for that page.

Taking the smallest published figure in each dimension gives the answer. The longest side is capped by United, JetBlue and Breeze at 17 inches. The middle side is capped by United at 10. The depth is capped by Frontier, American, JetBlue and Breeze at 8. A bag of 17 x 10 x 8 inches, measured with handles, wheels and straps included as Frontier requires, fits all six operating carriers above. In the airlines' own metric conversions that is roughly 43 x 25 x 20 cm.

That is a small bag. It is a slim backpack or a laptop tote, not the squat duffel sold as a Spirit special. The trade is clear. A traveler who flies Frontier and nothing else can use the full 18 x 14 x 8. The bags built to that size are compared in our 18x14x8 personal item picks. A traveler who mixes carriers on one itinerary, a Frontier outbound and a United return for instance, is bound by the smaller figure on every leg.

The usual suggestion, to overstuff a soft bag and squash it into the sizer, runs into Frontier's own wording. Straps and handles count, and a compressible bag that bulges past 8 inches deep has failed the same test as a rigid one.

The strongest objection is that the published numbers overstate the risk. Gate agents on full-service carriers rarely measure a backpack, and Luggage Wiz reports that some airports are removing metal sizers and returning to agent judgment. On American or Southwest, a bag half an inch over the limit usually goes through.

That is fair for the full-service carriers, and it does not change the conclusion for the budget ones. The same Luggage Wiz piece, written in late September 2026, says the real change this year is enforcement: more sizer frames at more gates, wheels and handles counted, and gate-check fees commonly between $50 and $150 or more, which is above the standard checked-bag fee. Budget carriers are described as the strictest of all. On a Frontier fare, a bag that is "probably fine" can add $100 at the gate, which may be more than the ticket cost.

Since July 2, 2026, a Department of Transportation final rule has let airlines and online travel agencies drop bag fees from the first page of fare search results. Disclosure is back to the 2011 standard, and a proposal to clarify personal item fee disclosure was dropped. The fare on a comparison site therefore says nothing about what fits under the seat or what the gate charges, which makes it worth pricing bags before choosing a fare, as set out in our method for finding cheap flights. One safeguard remains. A carrier that changes its baggage allowances must post the change on its homepage for at least three months, so a limit that moves between booking and departure should be visible to anyone who checks.

The practical rule follows from the arithmetic. A traveler buying one bag for trips on more than one airline should buy a bag of 17 x 10 x 8 inches or smaller. One who flies only Frontier can use the full 18 x 14 x 8. Anyone booked on Allegiant, Delta, Alaska or Sun Country should look up that airline's figure before buying either.