I'm looking for either an online calculator or the formulas to calculate how much concrete I need I'm pouring footings 24"x24" at 8" thick. I'm also pouring 8" piers that will be around 7' (max I hope) and I need to figure out how much concrete to get. how much water per 80lb bag should I figure on? We'll be hauling the water. does anyone know where I can find sonotubes that long? I've found them only in 4' lengths locally.