On Frontier Airlines in 2026, the only bag you can carry on for "free" is a single personal item. This item must fit entirely within the sizer box, which has maximum dimensions of 14 inches tall, 18 inches wide, and 8 inches deep (approximately 35 x 45 x 20 cm). This typically includes small backpacks, purses, or laptop bags. It is crucial to note that Frontier is notoriously strict about these measurements; if your bag has wheels or handles that protrude beyond the 18x14x8 limit, it will not be allowed as a free personal item. Any bag that exceeds these dimensions, such as a standard "carry-on" suitcase meant for the overhead bin, will incur a significant fee, which is often much higher if paid at the gate rather than during the online booking process. To avoid extra costs, many frequent flyers in 2026 use specialized "under-seat" bags designed specifically to maximize these exact Frontier dimensions. Remember that "free" does not include any overhead bin access; if you use the bin, you must pay for a full carry-on bag, which also has its own size and weight restrictions (usually 24x16x10 inches and under 35 lbs).