Bag manufacturer Maxwell Scott Bags recently released this little nifty tool that outlines carry-on restrictions for over 250 airlines. I'm pretty sure it's got most people's needs covered.
Because, like, paying for check-in is my favorite thing ever. The most useful part about this little tool is that it shows length, width and weight restrictions in the imperial or metric system. (Not all of us are under American command.)
It'll also note any additional restrictions for carry-on. For instance, the United Airlines blurb notes an additional:
1 X small bag or briefcase measuring 22 cm x 25 cm x 43 cm may also be carried on-board.
(Selecting the imperial or metric measure doesn't change the additional notes, just the main carry-on bag dimensions.)
A company rep said that it's updated regularly every time policies change. Check it out below: