Frontier's official size FAQ gives an 18 × 14 × 8 inch maximum for a personal item. Handles, wheels and straps count, and the bag must fit under the seat. This is a three-side box, not a 40-linear-inch allowance: an item can have an acceptable total and still be too deep for the sizer.
The three different bag products
| Item | Published size box | What to check about cost |
|---|---|---|
| Personal item | 18 × 14 × 8 in | Included under the ordinary policy when it fits |
| Carry-on | 24 × 16 × 10 in | Fare or bundle may include it; otherwise purchase terms apply |
| Standard checked bag | 62 linear in and 40 lb | Base price is itinerary dependent; excess is separate |
The last two rows are not upgrades automatically granted when the personal item fails the sizer. At the gate, a too-large underseat bag may become another product with a different charge. Check the current booking's included items before deciding how to pack.
Example: equal totals, different fit
A packed bag measuring 18 × 14 × 8 inches sits at the published personal-item box. A bag measuring 20 × 12 × 8 inches has the same 40-inch sum, yet its 20-inch side exceeds the 18-inch side of that box. Rotate the bag in the physical sizer if permitted, but compare all three outside dimensions; do not rely on total inches alone.
A backpack’s straps may compress, but a rigid laptop sleeve or handle may not. Measure the bag after packing it. If it’s close to the limit, save your booking and any written size guidance. The personal item checker compares the published box, but the gate agent still decides whether your packed bag fits.
Frontier's checked-bag weight threshold is a separate question. If you plan to move items from the backpack into a checked suitcase, see the Frontier 40 lb guide before assuming the transfer saves money.
Airline sources
Sources last checked 2026-10-31. Match the fare and flight in your booking before you pay or pack.