Once more, carry-on requirements may vary based on the aircraft type being used on an individual flight—smaller regional jets will have more restrictive carry-on rules than wide-body airliners. These standards are also subject to change at any time, so it's always a good idea to check with the airlines prior to travel, so you don't get caught by any last-minute rules changes.
As consumers, it's important to be wary of products claimed to be "FAA-approved" or "Government-approved" carry-on bags — there's no such thing. Federal agencies issue guidelines (see FAA Carry-On Advisory), but it's up to the individual carriers to set the rules.
The over-arching guideline — with very few exceptions, like canes and baby strollers — is anything you take on board must fit in an overhead compartment or under the seat in front of you, and that varies depending on the type of plane you're on. Additionally, every airline states that carry-on items are subject to a space-available limitation — you may have to check your carry-on bag if there's no room available. Hedge your bets by ensuring your carry-ons will fit under the seat in front of you.
That said, there are some general guidelines that can help you to select bags that are carry-on kosher. Obviously, giving these limitations a wide berth increases the likelihood that your new luggage won't be made suddenly obsolete by new regulations.
The maximum size for a carry-on is determined by each airline and the size of the aircraft, so make sure to check with them before flying. In general, most airlines have adopted a maximum carry-on size of 45 linear inches, determined by adding up the length, width and depth of a bag (typically 22 x 14 x 9 inches). These dimensions are determined by taking the longest measurement possible, so if your prospective carry-on has protruding handles and wheels they should be included in your calculations.
Some "wide-body" roll-aboards currently on the market measure 20 x 16 x 9 inches, which do meet the commonplace 45 linear-inch rule, but won't fit into the 14 x 9 x 22-inch sizing boxes typically used to determine whether bags meet carry-on dimensional requirements—which means they may often have to be checked at the gate. If in doubt, contact your air carrier, or stick with the 20 x 16 x 9 standard.
Be aware that overfilling a bag — especially one with protruding outside pockets — can turn a dimensionally legal bag into an oversize one that will have to be checked.
While most airlines do not weigh carry-on items, most have a de facto 40-pound limit per piece.
On domestic flights, the typical allowance is one carry-on bag, and one personal item. For most air carriers, that personal item is taken to be a purse, briefcase, laptop bag or camera bag. In almost every instance this personal item must be smaller than your allotted carry-on (you may not, for example, have two 22 x 14 x 9-inch bags and count one as a personal item). Some airlines set size/weight limits for personal items, so check with your airline.
* It is always recommended to check with your carrier prior to departure.