Derek Broughton wrote:
I treated it as an exact cylinder - if I wanted to figure out the difference
due to curvature, I'd actually have emptied the contents into a proper
measuring container, as otherwise I'd have to take into account the
curvature on the bottom, the tapering on the top, the size of the neck (at
least, the one I used had some water in the neck), and the thickness of the
plastic (which looks like it probably isn't even uniform). It's a lot
easier to make assumptions :-) (besides which, since I couldn't get an
exact measurement, it was all rough anyway). In fact, either our water
supplier is cheating us, or the jug is actually a little bigger than my
calculation, because it should really be about 1150 cu.in. for a 5 (US)
gallon container.

Assumptions make for easier math for sure, but with a little calculus and
angle measurements and the like you could calculate it precisely. I mean
if you wanted to take the time that is. No need for emptying and filling
or calibrated measurements of volume that way. Just a precise mathematical
description of your container and some number crunching. But for general
purposes particularly when it comes to fish, I agree, making assumptions
or rounding a bit usually is okay. Just don't make too big of assumptions
if dealing with something like medication.

As to the OP, if you are having trouble conceptualizing the cubic foot
holding 7.5g of water, look at a standard 5.5g and 10g aquarium. A 5.5g is
about 16" x 8" x 10" ~= 0.75 cubic feet. A 10g is 20" x 10" x 12" ~= 1.39
cubic feet. The reason the 5g water jug looks bigger than the 5.5g tank is
just dimensions. It's tall and narrow with a big dent for the handle (at
least with my water company), which gives the impression of being bigger
than a cubic foot. I seem to remember reading a study where humans thought
a tall, narrow item held more when compared to an equal volume sized
short, round item. I believe the study was related to overeating due to
underestimating the volume of what one was consuming, but not positive.