What size luggage do I need?

These figures assume you wear your bulkiest layer and heaviest shoes rather than packing them, and that you are packing clothes rather than equipment. Both assumptions move the answer a long way if they do not hold.

Two things are worth noticing. The jump from a weekend to a week is large; the jump from a week to a fortnight is almost nothing, because beyond about seven days you are not adding clothes, you are adding laundry. And winter is a genuine category change rather than a seasonal adjustment - a coat, a fleece and boots can take twenty liters between them, which is half a carry-on. How to pack for a week in a carry-on in winter deals with that case specifically.

Capacity is your decision. Dimensions are not. Most major US airlines allow a carry-on of 22 x 14 x 9 inches including wheels and handles; Southwest and Frontier publish a more generous 24 x 16 x 10; Spirit publishes 22 x 18 x 10. Every major US carrier allows a checked bag up to 62 linear inches - length plus width plus height added together - with a standard 50 lb weight limit, and Frontier at 40 lb.

• Buy to the standard, not to the exception. A bag built to Southwest's 24 x 16 x 10 will not clear an American or Delta sizer. A bag built to 22 x 14 x 9 clears everything.
• Weight is a separate ceiling, and on international routes it bites first. Many international and low-cost carriers cap cabin baggage at 7 to 10 kg, which a heavy bag spends a third of before you pack it. Best lightweight luggage ranks on exactly this.
• Linear inches, not the longest side. A 30-inch case that is 20 wide and 12 deep is 62 linear inches and legal. A 28-inch case that is 22 wide and 14 deep is 64 and is not.
• Expansion zips void the measurement. Alaska explicitly counts expansion toward the carry-on measurement, so an expanded bag is a different bag as far as the gate is concerned.
• Check your own carrier, every time. The two reference pages on this site - carry-on size by airline and checked bag size and weight limits - carry every published figure with each airline's own page linked.

1. Measure the bag packed, standing upright, on a hard floor. An empty softside bag measures smaller than a packed one, and the packed figure is the one you will be judged on.
2. Include the wheels and the handle housing. Airlines measure at the widest points. On the Maxlite 5 that is the difference between 21 inches and 23.
3. Include external pockets, loaded. A bulging front pocket is part of the depth measurement, and Alaska explicitly counts expandable pockets.
4. Add the three numbers for a checked bag. Height plus width plus depth is the linear-inch figure against the 62-inch limit.
5. Weigh it. A luggage scale costs very little and removes the entire category of check-in surprise - best luggage scales covers the hardware.
6. Write both numbers inside the lid. Then you never have to do this again, and you always know how much allowance you have left.

If the bag is a few tenths over on a softside case, it will usually give enough to pass a sizer. If it is a hardshell, it will not - there is no pushing a rigid shell into a box it does not fit.

If you are buying a single bag to cover most of your travel, the honest answer is a 40 to 46-liter carry-on built to 22 x 14 x 9 inches. It covers everything from a weekend to a fortnight, it clears every major US carrier, and it never gets lost by an airline because it never leaves you.

• Published overall dimensions at or under 22 x 14 x 9, wheels and handle included. This is the single most important specification and the one most often misquoted.
• Empty weight. On a weighed international cabin allowance this is the limit that binds. Under 6 lb for a wheeled carry-on is good; a frameless pack is lighter still.
• Forty liters or more. Below that a week stops working. Above about 46, you are buying volume you cannot legally use in the cabin.
• A squared-off interior. Tapered bags waste the corners of the airline's box. A clamshell or split-case opening uses them.
• No expansion zip you will be tempted by - or the discipline not to open it before a flight.

Then add a second bag only if you genuinely need it: a personal item for the weekends, or a checked bag for the fortnights and the winters. Those three cover almost every trip anyone takes. The picks below are one of each.