Last week I posted about a mistake in the translation from Imperial to Metric units on a luggage website, and Cathy Campbell pointed out in the comments another possible error: that the length times width times depth doesn’t equal the capacity. In this example:

25×18×11 equals 4950, not the 4720 cubic inches of capacity advertised. Possibly this is due to the thickness of the materials, but I’m wondering how they take that into account if that’s what it is. It seems more likely to me that they’re using a formula to get capacity than measuring it physically, but I’m not sure what formula they’re using. Anyone want to try it out? (It might be trivial or complicated; I spent more time uploading screenshots than actually working on a formula, although I did notice some oddities in the process.)

Here’s one line of luggage in this brand (the same as above), with the metric units removed because, well, they’re not all correct:

Here’s another line (same brand):

And a third line in the same brand.

Anyone want to hazard a guess as to the (possibly existing) formula(s) (now, with added parentheses!)?