ok, lots of people keep asking this so I thought Id put it in its own thread!! here is how to work out how many litres of substrate you need for a given tank size. And also how to calculate your tank volume in total if you need to. the formula is: (length x width x depth) / 1000 = litres. so for a 60x45x50 with a substrate 5cm deep (uniform) 60 x 45 x 5 = 13500cm2 / 1000 = 13.5 litres. Hope this helps.