Can a backpack be a carry-on?

Yes. Every major airline defines its cabin allowance by dimensions, not by bag type, so a backpack that fits within the published carry-on limit is a legal carry-on - and one that fits the smaller under-seat limit is a legal personal item. No carrier requires wheels, and none prohibits a backpack as such.

The genuinely useful question is which of your two cabin allowances the backpack should occupy, because on most fares you get both: one carry-on for the overhead bin and one personal item for under the seat in front of you. Get that choice right and you fly with more usable space; get it wrong and you either waste an allowance or get stopped at the gate.

Backpack as carry-on, or backpack as personal item?

Limits are the carriers' published figures and differ between airlines - check yours in the tables linked below before you fly.

The rule that actually catches people

A backpack is compressible, and that is exactly why gate agents treat it differently in practice from a hard case. A 40-liter pack measures within the limit when it is half full and bulges well past it when it is stuffed - particularly in depth, which is the dimension a bin sizer is strictest about. A hardside carry-on cannot grow; a backpack can, and will, and it is the one that gets pulled aside.

Two practical consequences. First, measure your pack loaded, the way you will actually fly with it, not empty on a bedroom floor - the method is in how to measure luggage. Second, treat any external compression straps as part of the plan rather than as decoration: cinching them down before you reach the gate is what keeps a full pack inside the box.

Which airlines are strict about it

US legacy carriers cluster around the same 22 x 14 x 9 in carry-on standard and rarely weigh cabin bags, so a well-packed backpack is comfortably fine. The pressure comes from two directions. European and Asian low-cost carriers enforce a smaller box and often add a cabin weight limit of roughly 7 to 10 kg that includes the bag - and a loaded travel backpack hits a weight limit long before it hits a size limit. Basic Economy fares on US carriers frequently remove the overhead-bin allowance entirely, leaving you the personal item only, which is precisely the scenario where a right-sized backpack is the ideal bag. The specific numbers are in our carry-on size by airline and personal item size by airline tables.

What a backpack gives up

Two things, and they are worth naming before you commit to traveling this way. You carry the weight - all of it, on your shoulders, through every terminal and connection, which a wheeled bag does not ask of you. And a soft pack protects fragile contents far less well than a molded shell. In exchange you get stairs, cobblestones, curbs and crowded trains without the drag, and a bag that never needs the bin.