Let us suppose your tank dimensions are L = 8ft B = 6 ft D/H = 5 ft Then your volume of tank will be 8 x 6 x 5 = 240 ft³ Now 1ft³ = 28.31 liter, hence, total volume of tank in liter shall be 240 x 28.31 = 6794.4 liter. That means almost 6800 liter of water can be stored in the tank. Here is a quick video to show you How to calculate water tank capacity in liters?