Most modern backpacks have stretchy mesh water bottle pockets on each side. Presumably this is so the pack bearer can reach around and take out a bottle for refreshment and rehydration. I for one can’t ever seem to make that work. I use them for something else, far more useful to the backpacker traveler. I stuff them each with my rain jacket and light down sweater. Now these things I need ready access to.
You know the kinds of jackets I am talking about.
They usually can pack down inside their own pockets insides out. Small but still a bit of bulk in your pack. And when you need them, sometimes in a hurry, they have to be found and then unpacked from their own self contained pockets. Slowly.
And when packed that way they still have some air in them so they are a bit bigger than they need to be.
In many cases they will fit really well, stuffed in with very little air still embedded in them, right in those side pockets. (Frankly, I thought this was the obvious use for these pockets and not worthy of a post, but apparently my niece backpacked a summer in Europe and this never occurred to her.) They won’t be taking up precious room in the pack. And getting to them takes about a second by just pulling them out.
If some airline official at an airport says your bag is too wide for carry on dimensions because of them, then just pull them out and put them on until past that station.
Easy. You’ve just added about 5 to 10L to your 40L carry on bag.