The recommendation, by the International Air Transport Association, suggests an optimal carry-on size of 21.5 inches in height, 13.5 inches wide and 7.5 inches deep. That’s smaller than the maximum size allowed by many airlines. For instance, American, Delta and United Airlines all allow bags up to 22 inches by 14 inches by 9 inches.
This is the IATA press release. "Perfect-size" carry-on bag?
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