Originally Posted by
Sant
When I began flying in the 90s, the carryon size limit was ~26in. By the end of the 90s, it was reduced to the current 24in. As all the airlines which I flew dropped to the new limit, I was under the impression the FAA set the maximum size.
Most USA-based airlines do not set any weight limits for carry-on bags. The most common limits used by USA-based airlines seems to be a total linear dimension of 45 inches (115cm), with limits of length (22 inches), depth (14 inches) and height (9 inches). These are the same maximum linear dimensions defined by Qantas for most of its international flights. However Qantas also defines a weight limit of 7kg. The weight limit is an airline-specific limit and not a limit imposed by any regulatory authority as can be seen by the different limits used by different airlines operating on the same routes.
This, of course, makes it interesting when some Australian international airport terminals impose carry-on bag weighing before entering security screening (common at BNE and MEL and possibly other airports). I don't know if the airport security staff doing the weighing take into account each airline's different carry-on baggage weight limits or if they just impose a universal limit across all carriers.
In my experience, even flying Qantas, the airport security personnel weighing carry-on bags prior to security screening will usually allow up to 10kg, especially for someone carrying an express departures card.