Is the carry-on maximum weight limit applied at check-in or at boarding?

The airline rules I've just studied use phrases such as "maximum hand baggage you can bring on board", that seems sensible, the limits apply to what you can actually bring onto the plane.

However when those rules are checked varies considerably airline by airline. Some airlines have standard size measurement frames, and some of those have built-in scales to verify both size and weight.

I have seen those measurement devices at checkin, at the gate as boarding passes are checked and at the aircraft door.

In more than 30 years of air travel I have never actually had the weight of my hand baggage checked, but then I am careful to use hand-baggage that is conspicuously within the permitted sizes.

My conclusion: airlines do have different policies as to if and when they verify hand-baggage conformance of both size and weight. A cautious passeger should assume that checks could be carried out at any point up to boarding the aircraft.

I wryly note that I have seen no limits as to the dimensions or weight of one's jacket. A passenger might carry quite a number of extra items in their jacket pockets.

In my experience it varies a lot airline and seems to be a bit if a fad that comes and goes.

About 10 years ago, Lufthansa Group (LH, LX, OS, ..) were quite strict about it and my carry-on was weighed frequently. That could happen at check-in, at the gate, and also in-between: One time I was stopped during an airside connection between terminals. They pulled me over and weighed my carry on before allowing me to enter the departure terminal.

Another time I checked in in Hong Kong for a flight to Zurich. Even at the first class counter (I had Star Alliance Gold status at the time) I got a serious tongue lashing because my carry on was 400g too heavy. I pointed out that HKG is very hot and ZRH is very cold and hence my jacket was in the carry on and that I would be happy to pull it out and just wear it. The agent grudgingly relented but growled at me to be "more careful next time".

This being said, this seems to have mostly gone away and I believe that they only pull the scale out to disallow obviously oversized bags. Weighing is quicker than the arguing about if it can be squeezed into the sizer or not.

US carriers do not care and as far as I know never have.

The rules apply to what you bring on board.

Note that:

• Usually duty free purchases are excluded from the limit. This is sometimes spelled out, sometimes not. YMMV.
• Full-service airlines usually allow a lot of extra stuff in addition to just a carry-on bag. Low-cost airlines are usually a lot more limited.
• Rules vary a lot from airline to airline and sometimes even from route to route.
• Sometimes there are also limits to how many items you are allowed to take through security. This was especially true after the liquid explosives scare.
• There are limits to the size of the items you can get through security (they need to fit through the scanner).

Size checks are most often made at the gate, if only because lots of passengers skip check-in/bag drop, and those are the most likely to go over the limit.

Weight checks are a lot less frequent at the gate: they need to have a scale for that, and the scale needs to be checked and controlled on a regular basis to be usable in these circumstances.

Some airlines have a scale included in the sizer, though.

However, many airlines will have check-in agents check carry-ons when they see you, especially if they look quite large or they see you are straining to carry them. Since they have a scale right there it’s easy for them to check weight limits.

Some airlines add small tags to the items they have seen at check-in and have considered OK to bring on board (Air France does this, or used to at least).

Of course with the generalisation of self-service check-in, this becomes less systematic, though there are still agents around with their eyes open.

At the gate, enforcement already varies a lot for size, but for weight it’s a lot less frequent, because it’s a lot more difficult for an agent to visually detect what is over the limit and what is not (while they are pretty good at detecting what is oversize).

The carry-on limit is the set limit the airline allows you to have on the plane. So, it applies when you enter the aircraft. When is it checked and enforced are different questions.

Although many airlines simply do not enforce their own official limits weight limits, plenty do enforce them. The vast majority of times, it happens at check-in. This is the most logical place since there nearly always a scale to weigh checked luggage anyway, plus they can generally accept payment and more importantly are not as pressed for time as during boarding that should happen as efficiently as possible.

While I've never seen the policy written down, some airlines seem to weigh carry-on systematically one some routes and directions (During my last round-trip between Dubai and The Seychelles, I did not see them weigh any leaving Dubai but they weighed all carry-on returning from The Seychelles and they were very strict, I left one book on the counter to be with 0.1kg of the limit).

Most airlines that weigh carry-on do it according to the discretion of the agent. They will target more those bags that look large since they could also be heavy. Note that most airlines accept a personal item and the weight limit for carry-on is usually for the carry-on only but there are some that have a per-item weight or a maximum weight for both items together. Personally, I have flown on airlines with all three types of limits and it seems that budget airlines that are more restrictive perform more frequent checks.

Airlines could check during boarding (Some have apparently a scale built-in to the sizer but I have never seen one myself) but that could be highly problematic for the passenger. Gate-checking is often offered for larger carry-on but some items cannot be checked by regulation, so you'd have to get onto the aircraft with some loose items and waiting for passengers to extract out what they need onboard, what to take out (without any obvious place to leave them), or what to keep is bound so slow down boarding.

Only once, I have seen carry-on weighed at security, in Bolivia, for a small airport what only served by one airline. This was an unexpected local flight as I had to return from a bus trip that couldn't return due to road blocks. In this case, I managed to explain that it was impossible to comply since my carry-on and personal items contained more than the allowed limit of items (electronics mostly) that were not allowed in checked luggage. I don't expect every place to be lenient this way, so always strive to be within the official limit. For another trip, I had a custom jacket made so that I could pocket 5kg of camera gear plus a laptop, that would otherwise have been over the limit.