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Hand luggage is rarely measured and even less often weighed. It does happen and some airlines are very strict but the majority of airlines only weigh checked luggage.

Before boarding most flights, there is a sizer close to the gate. It is a metal contraption that has the dimensions close to the maximum size allowed for hand luggage. Usually there are two parts to it, one if to measure the Carry-on and one for the smaller personal item. Most airlines in the world allow both but some only one which is another question all-together.

People do not systematically use the sizer. Instead, the airline announces a few minutes before people board that the sizer can be used to check. When the flight is particularly full, they often walk around the waiting area and tell people whose bags seem very close or over the limit to check the sizer. A bag that fits within the sizer is let in, one that does not is checked. If you can squeeze it in, you are good.

Sometimes you are told that the flight is very full and there is not even room for all the allowed size for everyone. In which case, they offer to gate-check the luggage which is free of charge.

The bottom line is that weight is rarely checked and size seldom but the closer you are to the limit, the more chances you are of being checked. Most airlines are not bothered by being over by less than a kilo, but some are stricter. Westjet from experience is not so strict and I have never seen them weighing hand-luggage.

They seldom weigh carry-on baggage. Unless you look like you're straining to carry it, I doubt you'll get checked. Checked bags are weighed every time.

If you have something that looks oversize you may be asked to fit it in a cage-like test frame. This is easier with something soft like a backpack. Screen capture from this video.

My experience is that airlines that charge extra for any checked baggage are very strict about weight and size limits for all baggage but particularly for carry-on. Jetstar, the Australian carrier, frequently checks the size and weight of bags at the time of boarding and a passenger with oversize (doesn't fit the frame) or over weight (0.5 kg over the limit) will have the option of either leaving the bag behind or being charged for the excess. Excess charges are frequently far more if they are paid at boarding time (which disrupts the flow of passengers) than if you anticipate the real weight and size that you need and pay for it ahead of time.

Regarding weight:

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  If you go through a check-in desk (rather than online, mobile or kiosk check-in), then as they have the scale right there they could ask you to put the bag on the scale to check, especially if the bag looks a bit large or it looks heavy (you are straining to keep it on your shoulder, or when picking it up or dropping it down).

  
  
  

  This happened to me a few times back when I used to fly Aer Lingus on the CDG-DUB route and they had a 7 kg limit for cabin bags.

  
  


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  I remember some airlines having the same sizing device but with an added scale built-in, so they could check both size and weight at the same time:

  
  
  

  

  
  
  

  Don't remember having seen one of those recently though.

  
  

Regarding size:

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  Note that the sizing device varies between airlines. Some are just a bunch of tubes like the one on the picture posted by Spehro Pefhany, which leaves quite a bit of flexibility especially for "soft" bags, wheels, pockets, handles, etc. while others have rigid flat sizes, which leaves a lot less flexibility:

  
  
  

  

  
  
  

  Wheels, pockets and handles can make the difference between "it fits" and "it doesn't, go to the desk to check your luggage and pay the at-the-airport checked-luggage fare".

  
  

In general, low-cost carriers and any carriers with restrictive luggage policies or fares without checked luggage are more aggressive when it comes to checking weight and size limits.

Also, limits are enforced more strictly when the flight is full and/or it has more people carrying lots of hand luggage (e.g. around the winter holidays) as space is scarce.

It can be "seasonal". Sometimes an airline decides they have a problem and go nuts. Check the flyertalk forums for mainline airlines before going.

I had Air Canada weigh my hand luggage at the business (!) check in (this was either YVR or YYZ) and insist I get both of them under 10kg despite the combined weight was under 20kg... Then they stopped this foolishness, it lasted a couple months. I think they even changed the rules so now it's just "you need to lift your bag" instead of "10kg".

I had American Airlines at one airport (sorry, too many years, too many airports, for my life can't remember which) have agents at the foot of an escalator leading to check in / security everyone caging their carry ons, tagging those that pass and there was a lot of anguish about this. This also happened only once.

I had British Airways at one point having people line up at the entrance of the gate (again can't remember which airport but it was IMO in Europe) and they caged everything and they didn't let in anyone to the gate area if it didn't fit. They even charged (heavily) for bags that didn't fit. It wasn't fun. This was a campaign of sorts of theirs, lasted a few weeks.

Low cost is different. Wizz Air always caged at Budapest, never at Luton. I frankly lost count how many times I did that in the last uh seven years or so. Bu then again they just changed Budapest airport so I am less sure. Especially because buying priority on these airlines not only allow you to carry two bags but makes you exempt from the blanket caging. I guess if you showed up with a 30" suitcase and tried to carry it on you'd be stopped but eventually you need to lift it up and put it in the luggage rack or underseat and if you can't you need to check it in anyways so there's a point where all this is futile.

In my experience it is entirely random, both whether the airline actually checks and which bags are deemed too big. The sizer is rarely used, nor is weighing the bag.

I've even had the bad experience of discussing with the gate staff about necessary medication in my carry-on they wanted to check. It was a soft gym bag about half full but it was still 'too big'. The people in the boarding line right in front of me both had much larger hard shell carry-on bags with wheels and everything and that was let on board no questions asked.

This is from my experience, which is still limited (only European lines, some standard, some low-cost, >90% of my trips had hand luggage only)

Weighing

If you have both checked and carry on luggage, usually both are going to be weighed at the check-in/luggage drop.

If you only have a carry on and check-in online you have small chances to have your luggage weighed, however it might not be true if you apparently struggle with it. My hand luggage has never been checked in such circumstances.

Even though you haven't asked - checked luggage is always weighed at the check-in/drop off.

How much can I stretch

Well, assume anything over the limit will be charged or rejected if captured. I don't know what is the accuracy of the scales.

How can I increase my chances

Check in on-line, don't use checked luggage. If you have to either check in a luggage or use a check in counter, leave hand luggage with a friend while dropping your checked luggage but it's not guaranteed. You may be asked if you have a hand-luggage with you and requested to weigh it anyway.

You may use a wheeled bag to conceal weight but it may not work as well.

Of course the most secure approach is to keep within limits.

Size

As others have already mentioned there are those boxes to measure your hand-luggage size. If it is measured and fits, it passes to the plane. If not there are up to 3 options: luggage stays behind, pay excess or send as a check-in. Availability of last options depends when you do the measurement and airline policy.

Now in my experience the luggage was controlled this way just once or twice for me (so a rate of 2-3%), on a low-cost flights only and not even all of them. The size of my bag was then carefully chosen and the bag went in leaving some 1-2mm spare only (on each dimension).

How much can I stretch

I have travelled with a soft bag that I knew was slightly oversize on a regular line plenty of times and never had it checked. I've seen people with even bigger luggage go on-board without any problem.

But I've heard from fellow travellers that rules depend here. Some airlines on some flights enforce far more strict policy. They measure almost every bag that might be close to the limit and nothing goes by if doesn't squeeze into the measuring box. Moreover I've heard of one airline who refused squeezing - the bag had to fit itself without any pressure. I've been flying with the same carrier though and never experienced anything that restrictive myself.

So on a regular line you can usually stretch to some limits (probably few cm will go through). It may also be that the dimension not physically limited in the measurement box is treated with more flexibility but the other two dimensions have to fit accurately.

On a low-cost assume that nothing extra will pass if checked and check is quite possible.

How can I increase my chances

Pick regular lines over low-cost.

Use soft bags rather than rigid and put soft somewhat compressible thing on luggage sides to allow squeezing into the measurement box. An alternative (IMO better) is to use a rigid case that fits into the dimensions and squeeze things inside of it.

Find out which size of the luggage is unbounded during measurement and stretch there.

Again - fitting to the limits is the most foolproof method.

If you're uncertain about size of your bag you can go to the airport and either find a measuring tool yourself or ask at the airline counter. Make sure to use the tool for the correct airline as the dimensions differ significantly. You will also see if your bag just fits (in which case you should not overpack it) ar there is a bit extra space allowing you to squeeze in that additional pair of...

Bottom line reads

On flights with full load the chance of more strict approach is higher as all the luggage might not fit into the cabin.

The limits are there for a purpose. Actually there are three of them:

1. Squeezing the bag into place for it on a plane (size)


2. Balancing the plane (weight)


3. Getting some extra bucks (both on low-cost airlines)

While third point one can treat as a rip-off, the other two are serious concerns. So rather than looking for ways to outsmart the system it's really better to keep to the limits.

Since this will be a vacation you may be wanting to bring back more than you took due to souveniring, so even if you make it to your destination, you may get dinged on your return as already pointed out in this comment. You should be prepared for that contingency; either be willing to surrender your sourvenirs or an equal weight of dirty clothes to the garbage bin, or have a collapsable fabric zippered bag that can be checked and "extra money" to pay the fee.

Overweight bags may be subject to a charge, which to the airline is also revenue for them, rather than purely punishment for you. So when packing to go home, ask yourself "Do airlines like their revenue?" followed by "Am I feeling lucky today?"

It depends massively on the airport.

I'm a frequent flyer and have seen my share of airports. Some don't even glance at your carry-on and some both measure and weigh it. My home airport makes me put my luggage on the scales about 2 out of every 3 trips I make, but they don't measure the size. Other airports check the size but not the weight.

It's really erratic and since you are a novice, you don't know your airports yet, so you should err on the side of caution or they'll make you check the bag if it goes over the stated limits. If you are within the stated limits, but the flight is full, they can also ask you to check the bag, but in that case it will be for free.

It is in your best interest to check that size and weight are within the limits.

If you exceed the size limits, the bag will not fit in the overhead space, so you will have to place it below the seat in front of you. It will still need to fit there, and it will be uncomfortable throughout the entire flight as it takes away from your legroom. Getting in first to claim your space does not help with that, flight attendants will rearrange the overhead space if it becomes full, and oversize items will be moved to the floor first.

If you exceed the weight limit, it is no longer legal to store overhead, which has the same result as being slightly oversize: you need to store it in the space where your feet would go if you were to sit comfortably.

Once I brought a wheeled carry on bag that weighed about 40 kg ( 90 lbs ) with me from USA to Europe. I didn't know about the weight limit. Before I boarded the flight, I found out about the weight limit. Because I didn't want the overhead compartment to break, I kept the bag beneath the seat on the floor in front of me, and didn't have any problems.

It is important to note that if anyone besides me had lifted the bag, it would have been obvious to them that it was a very heavy bag over the limit. Thus, if they had forced me to check this bag through to my destination, they would have found out.

This is important because sometimes on full flights they require you check your carry on bags at the gate. Lately in North America, this happens frequently. If they had forced me to do a gate check, my obviously very heavy would have been denied at best, and at worst hurt an airport employee.

My advise is to keep your carry-on close to the weight limit, just in case you are required to go a gate check. A few kg over might be ok, but don't pack the bag very heavy.

I live near Gatwick and use it a lot. Measuring carry-on bags seems pretty rare but it's down to the gate staff. Most gates do have those measuring devices for the relevant airline.

Weighing carry-on bags I have never seen. Unless it's so heavy you can't lift it into the overhead bin on the plane unaided I highly doubt you'll have any trouble.

With only a carry-on bag you won't need to visit the check in desks, just check in online and head to departures. At departures, Security won't care what size or shape your bags are as long as you can get them into the plastic trays and it fits through the scanners (and the scanners at LGW are much bigger than the overhead bins on the plane). Although they have machines checking boarding passes before security, the actual security staff don't associate you with your pass and don't know or care what restrictions your airline has. I actually had an odd problem where they confisticated a very small and not at all sharp tool I had in my bag. It would have been permitted on board by the airline, but LGW has a different set of rules which override the airline (despite what their website says).

Your risk is when (if) you return from Calgary. They might be much tougher there. I regularly see arguments at AMS between the gate staff boarding Easyjet flights who won't let pax board with oversize or extra carry-ons, with the pax arguing that the outbound flight from LGW permitted it. Usually they charge quite a lot and check the bag at the gate.