If you look at the top of the page you'll see the link for calculators.. I believe there's one for sand beds in there too.
, it has to do with the footprint of the tank not the volume. A tall column tank could also be 55 gal
but obviously would need much less sand than yours for the same depth.
Hmm, the calculator on this site gives a totally different result to the one at garf.org.. is it just me or does anyone else get that too? Is it because one is for sand and the other for aragonite?