Typically, the bag must be within the cabin baggage dimensions of around 22 x 14 x 9 inches (56 x 36 x 22 cm), commonly considered the standard size for carry-on. Any bag within this dimension can fit most aeroplanes' overhead compartments.

The size of a rucksack you can take on a plane in 2026 depends on whether you intend to use it as a "carry-on" or a "personal item." For a standard carry-on, most major U.S. airlines (like Delta, United, and American) allow bags up to 22 x 14 x 9 inches (approx. 45 linear inches). A rucksack with a capacity of 40 to 45 liters typically fits these dimensions. If you want to use the rucksack as a free personal item that fits under the seat, it must be much smaller—usually around 18 x 14 x 8 inches, which corresponds to a 20 to 25-liter daypack. In 2026, gate agents are increasingly strict; if your rucksack is "overstuffed" and doesn't fit in the metal sizer, you may be forced to pay a gate-check fee that can exceed $65. For international travel, particularly on European or Asian budget carriers, weight limits of 7kg to 10kg (15-22 lbs) are strictly enforced, so it is a high-value strategy to weigh your pack before leaving for the airport to ensure your 40L rucksack isn't deemed too heavy for the cabin.