Three bags, three capacity claims: 45 liters, 2,775 cubic inches, and enough for three days. None of those can be compared with either of the others as written, and two of them may not be measurements at all. Capacity is the spec that looks most useful on a luggage listing and survives scrutiny least well, so it is worth knowing how to convert it, how to sanity check it, and when to ignore it entirely.
The conversion, and the test it makes possible
One liter is 61.02 cubic inches. A 40 liter bag is a 2,441 cubic inch bag. A 3,000 cubic inch bag is a 49 liter bag. Putting every listing into one unit is the first step, and it takes a moment.
The second step is the one that catches errors. Multiply the three exterior dimensions together. That gives the volume of the smallest box the bag would fit inside, and the interior capacity has to be smaller than it, because shells, linings, wheel wells and handle channels all take space. A stated capacity larger than the exterior box is not an optimistic figure. It is a wrong figure, and it tells you the listing has not been checked.
What that test finds
The aluminum framed case in the Feilario carry on review publishes dimensions of 13.66 by 8.98 by 20 inches, which is an exterior box of about 2,453 cubic inches, and a capacity of 2,775.5 cubic inches. The claimed interior is roughly 13 percent larger than the whole bag. It is a small enough gap to slip past a reader who is not multiplying, which is exactly why it is worth multiplying.
Other gaps are not subtle. The case in the American Tourister Stratum 2.0 review lists 15 by 9.5 by 22 inches, an exterior box of 3,135 cubic inches, against a stated capacity of 8,125 cubic inches, which is more than two and a half times the bag. The expandable case in the U.S. Traveler Aviron Bay review gives exterior figures around 22 by 14 by 8 inches, roughly 40 liters of total box, and a capacity of 119.73 liters. The kids’ case in the Kiddietotes ride on luggage review measures 19.5 by 13.5 by 8 inches, about 34 liters of box, and claims 110 liters. The underseat case in the Kenneth Cole Chelsea review publishes overall dimensions of 17 by 13 by 9 inches, about 33 liters, against a stated 88 liters.
In every one of those cases the dimensions are plausible and the capacity is not. Where the two disagree, trust the dimensions, because dimensions are what get measured and capacity is what gets typed.
When capacity is just length times width times height
There is a second and more insidious pattern, where the capacity figure passes the test because it is derived from the very box it is being compared against.
The set piece in the Rockland softside luggage set review lists dimensions of 8 by 13 by 19 inches, which multiply to 1,976 cubic inches, and a capacity of 32.4 liters, which converts to 1,977 cubic inches. Those are the same number. The capacity field has been filled by multiplying the outside of the bag, so it describes a solid block of the bag’s shape rather than the space inside it. Around thirty listings in this catalog show that signature, matching their own bounding box to within two percent.
A figure produced that way is systematically too high, usually by a fifth or more once the shell, lining and wheel housings are subtracted. It is not useless, because it is at least proportional to the bag’s real size, but it cannot be compared against a figure that a different brand measured properly.
Capacity fields that hold something else entirely
Sometimes the field does not contain a volume at all. The bag in the Travel Select Amsterdam garment bag review lists a capacity of 1,200 tons. Elsewhere in the catalog the capacity field holds weights: pounds on several sets and kids’ cases, kilograms on a garment bag. Those are not exaggerations, they are the wrong field, and a listing that has put a weight in the capacity slot has told you something useful about how carefully the rest of its data was entered.
Garment bags have a further complication, since a hanging bag’s usable capacity depends on hanger count and fold depth rather than on a single volume figure. Anyone comparing options in the garment bags category gets more from the number of suits or hangers a maker claims than from any liter figure.
What a correct capacity statement looks like
Dakine publishes the model version on the bag in the Dakine Split Roller 110L review: 6,710 cubic inches and 110 liters given together, alongside exterior dimensions of 32 by 17 by 13 inches and a weight of 10 pounds. The two volume units convert into each other correctly, and the stated interior sits below the exterior box of about 7,072 cubic inches. Every figure on that listing can be checked against every other figure, which is all anyone should be asking for.
The trip length claim
Then there is the softest number of all. The underseat case in the RAINSMORE underseat carry on review says its main compartment holds two to three days of clothes, from a bag whose stated exterior dimensions work out to roughly 24 liters in total. The backpack in the Victoriatourist travel backpack review says three to seven days from a stated 40 liters.
Read together, those two listings imply somewhere between six and thirteen liters of packing per day. That is a factor of two between two bags in the same catalog, and it is the honest measure of how much a days claim is worth. It depends on the climate, on whether the trip includes laundry, on whether shoes and a jacket are in the bag or on the traveler, and on how bulky the garments are. A week of shirts in July and a week of sweaters in January are not the same volume.
Treat a days figure as a rough class marker rather than a specification. Under 25 liters is an overnight or a personal item. Around 35 to 45 liters is the usual cabin case range. Above 90 liters is checked luggage. Those bands are useful for narrowing a list in the carry-on luggage category, and they are all a days claim is really telling you.
How to compare two bags honestly
Convert every capacity to one unit. Multiply the exterior dimensions and discard any capacity that exceeds the result. Check whether the capacity equals the bounding box, and if it does, treat it as an upper bound rather than a measurement. Where a listing gives both a packing dimension set and an overall set, compute capacity from the packing figures, since that is the box that holds clothes. And where a listing gives a capacity but no dimensions, ignore the capacity, because there is nothing to check it against and no way to know whether the bag will fit anywhere.
