Is the 55x40x20 carryon limit for Ryan Air strict for each dimension or can it be a total of the three? For instance, could I have a a bag that is 50x45x20?
I think most airlines say it's one piece with dimensions not exceeding 115 cm total, but Ryan (website) says "maximum dimensions of 55cm x 40cm x 20cm is permitted" and they are just looking for a reason to charge you more. They use a frame with those dimensions, so if you bag is flexible (not a roll-aboard) and not packed to bulging, it might squeeze in. But just to be safe, find something that fits their dimensions.
And be VERY careful about the weight. The carry-on weight limit is 10 kg, about 22 pounds. SOme places are strict about it, they weight your bags (and lately they've been doing it while you're standing in line to get on the plane) and some places aren't so strict, but you never know which is which. Ilive in Germany and I travel by Ryanair a lot, so I know what I"m talking about. ALso, when they say one carry-on they mean one-I've been asked to put my (always small) purse in my bag.