20 inches is 1 foot 8 inches, or 50.8 centimeters. A useful exact-product example is the Arca Carry On suitcase, whose manufacturer lists its height as 20 inches (50.8 cm), with a width of 13.7 inches and a depth of 9 inches. That makes its height a concrete object comparison, not a claim that all carry-ons are 20 inches tall.
For a 22-inch reference, Samsonite lists its UpLIFT Hardside Carry-On Spinner at 22 inches high × 14 inches wide × 9 inches deep. The two heights differ by 2 inches (5.08 cm). A suitcase’s marketed size may refer to one dimension; check the complete outside measurements when comparing bags.