How the calculator works
It starts with a fixed allowance of about 12 liters for things every trip needs: a toiletry kit, sleepwear, a charger, and one layer. Then it adds space for each day's clothes, about 5 liters a day for light packers, 7 for average, and 9 for generous. After five nights it assumes you'll rewear or wash a few things, so each extra night adds half as much.
Cold weather adds about 6 liters for sweaters. Spare shoes add 5, a smart outfit adds 4, and a laptop adds 3. Finally it adds a quarter on top, because a soft duffle never packs as tightly as a box. The result is rounded up to the nearest 5 liters, which is how bags are usually sold.
How the carry-on check works
Each airline's size limit is turned into a volume. A soft duffle can't fill every corner of an airline's sizer box, so the calculator treats a bag as a comfortable fit when it's no bigger than about 85% of that volume. Up to 100% is marked tight: it will usually go in if you don't overstuff the ends. Anything bigger should be checked.
Weight is a separate question, and it matters more for leather than for nylon. An empty leather duffle weighs 1.5 to 2.5 kg before you put anything in it. The packed weight shown here is a rough estimate, so weigh your bag if your airline has a strict limit.
The short version
For most people, a duffle of 35 to 45 liters, about 20 to 22 inches long, covers almost every trip and still fits in the overhead bin. Our duffle size guide explains how to convert inches to liters, and our carry-on guide covers what gets a soft bag pulled aside at the gate.