Translation:How can one know the volume of a box?
I would rather invert, and write "Comment peut-on..." but perhaps that's just me.
In my limited experience with French, comment and pourquoi are the two interrogative words wherein there seems to be the most freedom to NOT invert. Is this your experience?
I think "combien" is as versatile in placement as "comment", but "pourquoi" is only very rarely at the end of a question:
- How can one know? = Comment peut-on savoir ? Comment on peut savoir ? On peut savoir comment ?
- Why does one want to know? = Pourquoi veut-on savoir ? Pourquoi on veut savoir ? (*)
- How much can you pay? = Combien peux-tu payer ? Combien tu peux payer ? Tu peux payer combien ?
(*) "On veut savoir pourquoi ?" means "Does one want to know why?" - not "Why does one want to know?"
To find the volume of a box, you can multiply it's height, width, and length.
If I heard or read this sentence in isolation, I'd think "an excellent non-native speaker of English, translating literally from their own language ... or perhaps a philosopher". I think the meaning, or perhaps better, the function, of this sentence is subtly different: it's the question of a maths teacher soliciting information from a class. In which case, the question you'd ask in (UK) English would be "How do /can you work out the volume of a box?". (For context: I'm a native English speaker who lived in France for five years; I'd say my French is fairly fluent but sometimes, er, wrong 8^)