Budget airline fares in the US now include one bag: the personal item that goes under the seat in front of you. Everything else costs extra. A backpack built to that size, usually 18 x 14 x 8 inches, is the difference between a cheap flight and a cheap flight plus a bag fee each way.
The bags that get people charged at the gate are the ones that only measure right when empty, bulge past the depth limit when packed, or are sold as 'personal item' without ever stating a size. The other common disappointment is a laptop sleeve that stops at 15.6 inches when your laptop is 17.
We compared five backpacks sold for exactly this job, all with their dimensions stated in the listing. One of them fits the under-seat limit, opens flat like a suitcase, holds a 17-inch laptop, comes with packing cubes, and unzips to full carry-on size on the trips where you have paid for the bin.
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Our top pickZOMFELT Expandable Personal Item Travel Backpack (40-45L)
Compared the dimensions each listing states against the common US under-seat limit of 18 x 14 x 8 inches, noting bags that only reach that size when loaded
Checked the stated laptop sleeve size, because 15.6 inches is common and 17 inches is rare
Looked for a 180-degree suitcase-style opening, a luggage pass-through strap and a wet pocket, as stated in the listing
Noted weight, warranty and included accessories as listed; we did not fly with these bags
ZOMFELT Expandable Personal Item Travel Backpack (40-45L)
Fits a 17-inch laptop
Expands to full carry-on size
Packing cubes included
Sized 18 x 14 x 8 for the under-seat allowance, with a 2-inch expansion to 22 x 14 x 9 carry-on size, a 180-degree opening, a 17-inch laptop sleeve, three packing cubes and a 1-year warranty.
Fits the 18 x 14 x 8 under-seat limit and expands by 2 inches to the standard 22 x 14 x 9 carry-on size when you have paid for the bin
Padded sleeve for laptops up to 17 inches; 12 compartments including mesh and wet pockets
The fixed-size version: 18 x 14 x 8 inches at 1.76 lb, 180-degree zipper opening, a 17.3-inch laptop compartment, wet pocket, elastic compression bands and a luggage strap.
18 x 14 x 8 inches and 0.8 kg (1.76 lb), the lightest bag in the group
Separate laptop compartment for a 17.3-inch laptop and a 13-inch tablet
180-degree zipper opening, wet pocket, elastic bands to hold clothes in place
Padded handles on top and side plus a luggage strap
Fixed size; there is no expansion for trips where you have a carry-on allowance
A 16.9 x 11.8 x 7.9 inch bag with hidden shoulder straps that tuck away to make a briefcase, a 16-inch laptop sleeve, a 180-degree opening and a 2-year after-sales promise.
16.9 x 11.8 x 7.87 inches, well inside the 18 x 14 x 8 limit, and the maker names Spirit, Frontier and JetBlue
Hidden shoulder straps tuck into the back panel to convert it into a briefcase; luggage pass-through strap
Padded compartment for a 16-inch laptop and a 12.9-inch tablet; wet pocket
High-density water-resistant nylon; 2-year after-sales service stated in the listing; about 950 g
16-inch laptop sleeve, so a 17-inch machine will not fit
Fixed 26-litre size with no expansion, and no packing cubes
Nine pockets across three compartments, a 15.6-inch laptop sleeve, a hidden anti-theft pocket, a USB charging port and a suitcase-style opening. Measures 18 x 14 x 8 when fully loaded.
Three main compartments with nine pockets, internal compression straps and a wet pocket
Suitcase-style opening; hidden anti-theft pocket; built-in USB charging port
Padded shoulder straps with an adjustable chest strap, mesh back panel and luggage strap
The maker names Frontier, Spirit, Southwest and JetBlue
15.6-inch laptop sleeve
Listed at 18 x 14 x 8 only when fully loaded (16.5 x 11.8 x 7.1 empty), so overpacking pushes it past the limit; fixed size; no cubes
The smallest bag here at 17.1 x 11.5 x 7.1 inches, with a 180-degree suitcase opening, a padded 15.6-inch laptop compartment and side compression straps to keep it slim.
17.1 x 11.5 x 7.1 inches, the smallest footprint in the group for airlines with tighter under-seat sizers
Main compartment opens 180 degrees like a suitcase, with a toiletries mesh pocket and stationery organiser
Padded 15.6-inch laptop compartment with an elastic retention strap
Side compression straps with buckles; two large exterior zip pockets
15.6-inch laptop limit and 25 litres is tight for more than a weekend
Compresses but does not expand; no packing cubes included
Why ZOMFELT Expandable Personal Item Travel Backpack (40-45L) is our #1 pick
One bag for both fare types
Zipped, it is an 18 x 14 x 8 personal item. Unzipped, the listing says it reaches the standard 22 x 14 x 9 carry-on size. You buy one bag and use the allowance you actually paid for on each flight.
Room for a real laptop
A 17-inch sleeve is unusual in this category, where 15.6 inches is the norm. If you travel with a large laptop, this and its fixed-size sibling are the only two here that take it.
Packing cubes in the box
The garment, shoe and toiletry cubes are what make a clamshell bag work: pack them at home, drop them in, and the bag opens flat at security without spilling. The other four bags leave you buying them separately.
American, Spirit and Frontier publish 18 x 14 x 8 inches for the personal item. United publishes a smaller 17 x 10 x 9. Southwest and Delta use their own figures or simply require the bag to fit under the seat. Pack for the strictest airline on your itinerary, and check its site before you fly, since these rules change.
Measure packed, not empty
Soft bags grow. A listing that gives one size empty and a bigger size loaded is telling you where the limit is. Keep the depth under 8 inches when packed and the bag will slide under the seat.
Check the laptop sleeve before anything else
Most personal item bags fit a 15.6-inch laptop. If yours is 16 or 17 inches, that single spec rules out most of the market; two of the five bags here take a 17-inch machine.
Decide whether you ever pay for the bin
If every flight you take is a basic fare, a fixed-size bag is lighter and simpler. If you sometimes fly with a carry-on allowance, an expandable bag saves owning two.
FAQ
What size is a personal item on most US airlines?
The common published figure is 18 x 14 x 8 inches (American, Spirit, Frontier). United allows 17 x 10 x 9. Others require the bag to fit under the seat without a fixed figure. Always check your airline.
Can a backpack be a personal item and a carry-on?
Only if it changes size. The expandable ZOMFELT is sized as a personal item when zipped and reaches carry-on size when expanded; the other bags here are fixed-size personal items.
Will a 17-inch laptop fit in a personal item backpack?
In most, no. Two of the five bags here state a 17-inch or 17.3-inch sleeve; the rest stop at 15.6 or 16 inches.
Do airlines actually measure personal items?
Budget airlines do, with a sizer at the gate, and charge for bags that do not fit. A bag that measures right when packed, not just empty, avoids the fee.
Bottom line
For most travellers the expandable ZOMFELT is the pick: under-seat size when you need it, carry-on size when you have paid for it, a 17-inch laptop sleeve and the packing cubes already in the box. Choose the fixed-size ZOMFELT if you want the lightest bag and never use a bin allowance, and the Sinaliy if the bag doubles as a work briefcase.
