Today I bought a "40" gallon Hex tank. The formula is not an easy one and estimating it is a nightmare. It was only $45 and in good shape. I wasn't really worried about the gallonage because its looks were good enough. It was no where near forty gallons btw. When I got it home (bought two tanks today) I did the calculations and it turned out to be 25g. I am still happy with it. I just can't decide whether to drill the bottom for a reef like tank or add a large sponge filter and some oscars (not sure if I want that kind of maintenance).
I later figured out a good estimate for hex tanks. Measure the radius from the center to a point 1/3 from any of the six corners. Then use that in the circle formula Pi*r^2, then multiply that by the height. I don't carry a calculator with me when I am not at school, so...