An editorial answer to a carry-on sizing question found during travel-backpack research.
1 answer
Choose a backpack that stays within the smallest allowance on your itinerary when it is packed with your actual belongings. Start with external measurements and any applicable weight limit. Liters help compare packing capacity, but they cannot tell you whether a tall, narrow bag or a short, deep one fits the permitted shape.
As one airline-specific example, Delta publishes a maximum of 22 by 14 by 9 inches for its main carry-on. That example is not a universal target, and it is different from the smaller item intended to fit beneath the seat. Your booking also needs to include the type of bag you plan to bring.
After settling the limit, choose the smallest capacity that holds your necessary kit without straining the closures. Try clothing, toiletries and electronics together; checking them as separate piles can hide the real space requirement. Measure loaded side pockets and any expansion section, too.
If you are choosing between two suitable bags, leave some tolerance rather than pursuing every last inch. That makes ordinary repacking easier and reduces dependence on an unusually neat first packing attempt.
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