The standard domestic carry-on luggage size is 22” x 14” x 9” (including the handle and wheels), though you might find an inch or two of difference between airlines—for example, Southwest allows bags up to 24” x 16” x 10”.
International carry-on sizes vary, but most follow the same size requirements as domestic airlines.
Note: If you see a size listed in “linear inches,” take the combined length, width and depth of the bag and you’ll have your total in linear inches. 22” + 14” + 9” = 45 linear inches.
To avoid getting nailed on the size of your carry-on, check the requirements when you book your ticket or consult this handy list.