A duffle bag can definitely be used as a carry-on, provided its dimensions fit within the airline's specific size limits. Most major airlines (like Delta, United, or British Airways) require carry-on bags to be no larger than 22 x 14 x 9 inches (56 x 36 x 23 cm). One of the biggest advantages of a duffle bag is its flexibility; because it is soft-sided, it can often be "squished" into a tight overhead bin or under a seat more easily than a hard-shell suitcase. However, this is also a risk: if you overpack a duffle bag until it "bulges," it may exceed the size limits and be rejected at the gate. Many manufacturers now produce "travel duffles" specifically designed with carry-on dimensions and internal structures to help maintain their shape. When choosing a duffle for travel, look for one with a padded shoulder strap or "backpack straps" to make navigating the airport easier. As long as you don't overstuff it to the point where it won't fit in the metal "sizer" at the gate, a duffle bag is an excellent, lightweight alternative to a traditional rolling suitcase.