On Frontier Airlines, you are allowed one Personal Item for free, and it must fit entirely within the dimensions of 14"H x 18"W x 8"D (35 x 45 x 20 cm). This typically equates to a small-to-medium school-sized backpack, a large purse, or a laptop bag. It is crucial to include handles, wheels, and straps in these measurements, as Frontier is notorious for using "sizers" at the gate in 2026 to ensure compliance. If your backpack is overstuffed and doesn't slide easily into the metal sizer, you may be forced to pay a "gate bag fee," which can be significantly more expensive (often $99 or more) than if you had pre-purchased a carry-on. To maximize your free space, many travelers use "underseat-specific" bags designed precisely to these 18x14x8 dimensions. Anything larger—such as a standard 40L travel backpack or a small rolling suitcase—is considered a "Carry-On" and requires an additional fee. To avoid a headache at the airport, always test your bag at home and ensure it is not "bulging" past that 8-inch depth limit.