Most US domestic airlines don't have a weight limit for carry on luggage, only a dimensions limits. However, the rule is that you must be able to lift your carry on into the overhead compartment by yourself or it should be small enough to stow under the seat in front of you. Some international airlines do have a weight restriction for cabin bags and a few even weigh yours at the gate. If it goes over the limit they can ask you to gate check it and pay the checked bag fee if there is one. Almost all airlines allow a personal item that should go under the seat in addition to your carry on.