For most airlines in 2026, a "small backpack" is defined as a personal item that must be able to fit completely under the seat in front of you. While specific dimensions vary slightly by carrier, a safe high-fidelity baseline is approximately 18 x 14 x 8 inches (45 x 35 x 20 cm). In terms of volume, this typically equates to a backpack between 15 and 25 liters. This size is distinct from a "carry-on" bag, which is larger and intended for the overhead bin. If you are flying on low-cost carriers like Spirit, Frontier, or Ryanair, they are extremely strict about these measurements; if your backpack is overstuffed and cannot fit into the metal "sizer" at the gate, you will likely be charged a significant fee to gate-check it. For a stress-free high-fidelity travel experience, look for backpacks labeled as "TSA-approved" or "under-seat" compatible. Soft-sided bags are often better than rigid ones because they can be "squished" to meet the required dimensions even if they are slightly over the limit, ensuring you avoid unexpected "extra baggage" charges.