40L is perfectly within the size limits for carry on as long as it has the correct dimensions. Many bags, even 40L bags, don't however. They go for looks or whatever, they think will sell better.

In 2026, whether a 40L backpack qualifies as a carry-on depends heavily on its dimensions and the airline's specific "Condition of Carriage." Most 40L bags (like the Osprey Farpoint or Tortuga Travel Backpack) are specifically designed to meet the standard international carry-on size of 22 x 14 x 9 inches (55 x 35 x 23 cm). However, if the 40L bag is fully stuffed, it often "blooms" in depth, exceeding the 9-inch limit and potentially getting flagged by gate agents. On budget carriers like Ryanair or Spirit, a 40L bag is almost always too large for the "free" personal item under-seat bin and must be paid for as a cabin bag. For major carriers like Delta, United, or Emirates, a 40L bag is usually acceptable as a carry-on if it fits in the overhead bin. To be safe in 2026, you should look for "travel-specific" 40L bags that have compression straps to pull the load tight, and always avoid overpacking the brain or front pockets, which can add those crucial extra inches that lead to a forced gate-check and unexpected fees.