Volume to weight - container question

I blame this board, of course, for the fact that I am now reconsidering my purchasing practices. The number of trips I'm having to make to the store to get more flour are starting to get annoying and I need to consider larger storage containers. This, of course, will also necessitate rearranging my kitchen - no small undertaking. I live in an apartment and, unfortunately, they saw fit to put in half-shelves in all the lower cabinets, so I don't have the full depth of my cabinets to work with.

The 40qt commercial bulk container available at The Container Store -almost- but not-quite fits in my cabinets. Unfortunately, the next step down is an 18qt, which also looks like it's about the right diameter to fit. The question is, how much flour would an 18qt container hold? There's four cups (roughly) to a quart - but I believe that's for liquid measure. Flour of course is very compact, so may be equivalent? If there's roughly 4.25-4.5oz per cup of flour, would that be about 19-20 pounds of flour, then?

I'm not sure how much I can save by ordering bread and rye flour by mere 10 pound bags, but at least I'd not have to make so many trips to the store.

My husband suggested getting the biggest container and storing them in the bathtub that we never use. Just draw the curtain closed and hide them. Oh, the mockery, even as he consumes the fruit of my labor!