greetings.
to be precise about how much gravel/sand (whatever you chose) you could do a calculation on how much volume you need to cover the buttom of the aquarium with, substrate products usually say how much volume that they contain.
i.e. lenght X height (how tall the gravel layer sould be) X depth / 1000 = liters, unfortunately i don't know how you americans calculate it.
as for weight, i really doubt you sould be worried. i've never heard of anyone having problems with adding gravel/substrate/rocks to their tank.