I've seen others mention this calculator before: http://www.garf.org/calculators/SandBedCalculator.asp. I've never used it though. What kind of substrate are you planning on using, because I'm sure that would affect how much you need.
if you dont have an undergravel filter, a shallow substrate is good. ive seen some people that dont have any substrate at all. deep substrate gets messy and could make nitrates rise.