You know, I've never heard a formula for calculating how much sand you need for a given size tank by the weight of it. I'm sure there must be one like there is for gravel.
Anyway, my posting on your thread will bring it back to the top of the list and perhaps someone can enlighten us both.
according to tankmaster(see sig)
fo a 1" layer of sand you will need 41.4 Pounds/18.77 Kilograms
this will displace 3.11 US Gallons / 11.77 Liters of water