The sizes for carry on luggage vary among airlines. But the largest you are usually allowed is 45” (22 x 14 x 9). The 45 refers to the total number in inches. It doesn’t matter if the figure is 40”, 50” or something else. The figure always indicates the combined height, length and width of the luggage.
The majority of travel luggage sold is in standard shapes. This is necessary to avoid problems. Some airlines do not accept irregularly shaped bags even if they are within the measurement limits. When shopping for luggage, take note of the dimensions.
There are many suitcases labeled as carry on. But some of them are beyond the limits of what airlines will accept. Again, the safest is 45”. But some airlines allow luggage up to 55”. However, this is quite rare. To be sure, check the airline. You cannot rely on the store.
Some of the products sold are over the size limit. Other times the measurements are wrong. Sometimes the numbers are only for the compartment interior. Airlines include external pockets in the total, so measure them too. If they are filled, it will add to the total figures. The carry handle and wheels have to be included in the calculations too.
On the Plane
As long as you know the sizes for carry on luggage allowed, there should be no problems getting them onboard. They are usually stored at the overheard bin. You can also put it under the seat. But this isn’t always possible. The areas under airline seats are not getting any bigger. In fact they are being reduced in most airlines. This is because of the electronic gadgets which are now on the seats. Another reason is the space between seats is being reduced.
The design of the supports and seat frame, the area under the center seat has the most space. The least is below the aisle seat. The area under the wing seat is somewhere between the two. However, the geometrical shape of these spaces varies. If your luggage is big, it might not fit. Because of these facts, it is more practical to use the smallest possible luggage allowed.
While the sizes for carry on luggage are limiting, airlines allow you to bring one personal item. This may be a laptop, camera, briefcase, small book-bag style backpacks or reading matter. Other items not more than 36” fall in this category.