The space under the seats of most airplanes is about 20 inches wide, 17 inches deep and 11 inches high. Some airplanes recommend that the sum of the length, width and height of luggage should not exceed a given linear length while others give exact specifications for carry-on luggage.
Delta Airlines stipulates that luggage brought into the cabins of its planes should not exceed a width of 22 inches, a depth of 14 inches and a height of 9 inches. American Airlines requires any baggage that goes under a set to be 45 linear inches or less. Continental Airlines allows carry-on luggage of up to 51 linear inches.