A backpack counts as a carry-on if it meets the airline’s carry-on size limit of roughly 22 x 14 x 9 inches. Smaller backpacks that fit under the seat in front of you count as a personal item instead.
The classification depends on dimensions:
- Personal item: Under 18 x 14 x 8 inches. Goes under the seat.
- Carry-on: Up to 22 x 14 x 9 inches. Goes in the overhead bin.
Most travel backpacks in the 30 to 40 liter range are sized to meet the carry-on limit. Smaller daypacks under 25 liters usually count as a personal item.
Fully-stuffed structured backpacks are less forgiving than soft-sided ones. If a frame pack looks larger than 22 x 14 x 9, gate agents will measure it.
See: airline baggage rules and the luggage calculator.