To calculate the amount of LS needed:
L x W x Ht (of the sand you desire)= cu. in.
Divide the product by 14(The volume of LS per pound).
Ex: You have 75G tank with L=48in, H=18in, and you like 4 inches of sand.
48 x 18 x 4 = 3,456 cu. in. Divide this by 14(cu. in. per pound of LS)=247 lbs...