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I'll soon do my first intercontinental flight. Due to price, I decided to travel with hand luggage only. There are specified weight and size limits, of course – in my case it's 7kg and 53x38x20cm on a WestJet flight from London (LGW) to Calgary (YYC).

As these limits are quite restricting for me, I want to at least fully utilize them. So from this angle, how seriously should I take these values?

1. Weight: Weighing is easy and quick so I suppose they will do it every time? How much "overweight" would be tolerable as a rule of thumb, if any?




2. Size: This seems a bit more complicated to me. If I give them a perfectly cuboid-shaped cardboard box, the surely can measure it without trouble, but what about a object with the irregular shape and elasticity of an overstuffed backbag? How do they determine if it's small enough? Do they have something like a 53x38x20cm sized box, and if the bag can be squeezed in it's fine?

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 go 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.

Regarding weight:

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  If you go through a check-in desk (rather than online, mobile or kiosk check-in), then as the have the scale right there they could ask you go 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 checked 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 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.