In 2026, the "standard" size for a carry-on backpack is 22 x 14 x 9 inches (56 x 35 x 23 cm), which is designed to fit in the overhead bin of most major jets. However, if you want your backpack to count as a "personal item" (to be kept under the seat and brought on for free even on "Basic Economy" tickets), it must be significantly smaller—typically around 17 x 10 x 9 inches (43 x 25 x 22 cm). Airlines have become much stricter in 2026, with many removing gate sizers and relying on gate-agent discretion, which often leads to more "surprise" gate-checks for backpacks that look too bulky. It is important to remember that these measurements must include all handles, wheels, and side pockets. If your backpack is overstuffed and won't fit in the "sizer" box, you may be charged up to $65 to check it at the gate. As a grounded recommendation, if you are flying a "Low-Cost Carrier" (like Ryanair, Spirit, or Frontier), stick to a 20–25 liter backpack to ensure it meets the "Personal Item" limit and avoids any hidden fees that could double the cost of your ticket.