I was wondering how many gallons my 20 long (30 x 12 x 12) + 30 pounds of sand actually holds. Hey, it's 4:30 am and I'm not sleepy.
Anyway, according to this site, 15 gallons. Pretty accurate since it took around 14 to fill it with two shrimp towers, two dividers, one filter and two heaters plus some Cholla wood. I didn't factor in the above when I used the calculator.
My five gallon (15 X 10 X 6.5) with 7 pound of gravel holds about 3.6 gallons. It is accurate almost exactly because I fill it with a bit over 3.5 gallons. It should be noted the published dimensions for the two Hawkeye 5 gallon kits are not accurate.