WE’VE become pretty clever about the weight of our luggage when flying.

We’ve had to. We don’t want to pay overweight luggage fees any more than we can afford them.

But something that’s sneaking up on travellers and catching them out — and I’m among them — is cabin bag size.

I’m talking about the actual dimensions of the bags we want to bring on board: the height, width and depth measurements, down to the millimetre.

In their endless grab for extra charges, airlines are getting increasingly strict about the size of carry-on bags. A mere centimetre over the limit could mean we’re charged a king’s ransom to stow it.

This became a problem during a recent trip to Europe where I (ambitiously) flew on seven flights across seven different airlines.

It’s no longer just enough to get your bag to fit in the overhead compartment — it still has to comply with the airline’s rules. Picture: iStockSource:Supplied

I’d thought out every detail of the flights but never thought twice about the trusty duffel bag I always used as hand luggage.

Size limits varied widely across those seven airlines, and generally, they seemed to be shrinking. On some carriers, like Emirates (where the cabin bag limit is 55 x 38 x 20cm), my duffel bag was in the clear. With others, like Wizz Air (40 x 30 x 20cm), which deemed my bag centimetres too long, I had no choice but to shove it and its contents into my checked suitcase and board with just a phone and wallet, or pay handsomely for a late bag check-in. This also added a fair amount of heft to the suitcase I was trying to keep underweight.The eve of every flight found me in mini-meltdown mode as I conjured up ever creative ways of avoiding anticipated crackdowns on my sometimes-OK-but-sometimes-not carry-on bag. I also had to buy a ruler for the first time in about 17 years.

This app might have made things a lot easier.

KAYAK.com.au has added a new “bag measurement tool” feature to its app, which scans and measures the bag in question and tells you which airlines will and won’t let you take it in the cabin.

The app uses augmented reality to measure the height, length and depth of your bag — and all you need to do is point and shoot with your phone camera.

“A common pain point when travelling is that moment when you begin to worry if your carry-on luggage is the right size for your flight,” said Matthias Keller, APAC KAYAK’s chief scientist and senior vice president of technology, with whom I wholeheartedly agree.

“With allowances for carry-on luggage differing between airlines, it can be hard to keep track. “KAYAK’s new tool was designed with this in mind. It uses augmented reality to measure luggage and also compares baggage fee policies, so travellers can ensure their bag is an appropriate size before they’ve even left home.”

I’ve tried the measuring tool about 10 times on three different bags and it’s a cinch to use.

After launching the tool on the app, you simply point your camera towards the floor, which calibrates measurements, and then you scan your bag.

KAYAK told me to avoid leaning the bag against a wall in order to ensure a clear read.

Dots and lines pop up over the image as the app calculates the bag’s dimensions.

After a few seconds, you’re told the height, width and depth in centimetres, and directed to an enormous list of international airlines.

Each airline is marked with a tick or cross, depending on whether or not your bag’s measurements comply with its restrictions. KAYAK says the list of airlines is expanding and updating.

On my first few tries, the app tool measured the width and depth of the bag down to the exact centimetre, with a small bit of discrepancy with the bag height, possibly because of the handles and straps.

You then get the results for the bag you just scanned, in a long, long list of airlines.Source:News Corp Australia

This feature can be brilliant when the results are 100 per cent accurate, because a couple of centimetres could be the difference between having to pay to stow the bag, or not.

I especially appreciated the list that compared airlines and their restrictions, because as I found during my pre-flight meltdowns in Europe, Googling this information produces conflicting, confusing and frequently upsetting results.

The tool is currently available in the KAYAK app for all iOS devices running iOS 11.3 and up. It’s free, so it’s worth giving a try — well before the night before your flight, I might add.