Like most travelers, you probably want to maximize your packing space. But you also don't want to check a bag. The solution is a large carry on backpack.

The best size to maximize your packing space while staying compliant with budget and European airlines is just under 22 x 14 x 8 inches (55 x 35 x 20 cm). These dimensions add up to just under 40 liters in volume.

In the US, you can have carry ons up to 22 x 14 x 9 inches or 45 liters. But that size of bag will only work in the US. To make sure you won't have to check your bag when flying internationally, keep it under 40 liters.

A 40L travel backpack will still be big enough to fit a week's worth of clothes, toiletries, and a laptop.