The ideal size for a carry-on trolley bag in 2026 is 55 x 40 x 20 cm (approximately 21.5 x 15.5 x 8 inches). These dimensions are designed to fit the most restrictive "sizing cages" used by major airlines like Lufthansa, Air France, and various low-cost carriers. This size ensures the bag will fit in the overhead bins of both large wide-body jets and smaller regional aircraft. If you are flying primarily with North American carriers (like Delta or United), you can often get away with a slightly larger 56 x 36 x 23 cm bag, but the 55x40x20 standard is the "universal" sweet spot for international travel. When selecting a bag, always ensure the wheels and handles are included in your measurements, as these are the most common points of failure during a gate-check. In 2026, many frequent flyers also opt for a "personal item" sized bag (40 x 30 x 15 cm) that fits under the seat, as this is the only bag guaranteed to stay with you for free on Basic Economy fares.