Most airlines allow backpacks as carry-ons, but size and weight restrictions vary depending on the carrier. 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.
Does a backpack count as a carry?
Domestic carry-on policies. A good rule of thumb is that if your carry-on bag fits under the seat in front of you, it's considered a personal item. This includes small backpacks, purses, briefcases, laptop bags, etc. Larger bags, including suitcases, must fit in the overhead bin compartment.
Is a backpack a carry-on or personal item?
In most cases, backpacks are considered personal items as long as they fit under the seat in front of you. For example, if you use a backpack as a personal item and put it under the seat in front of you, you can then use a duffel as a carry-on bag as long as it's under the airline's size requirements.