The 40-liter Patagonia Black Hole Duffel 40L is sized for carry-on compliance at most airlines. The 55L is technically too big, especially when fully packed, but some travelers have been able to use it as a carryon when it's not completely full. A related "Black Hole" model, the Patagonia Black Hole MLC 45L, is designed to most closely fit most airline carry-on specifications. Note: Standard carry-on sizing is approximately 22" x 14" x 9" which results in, when added together, about 45 linear inches.