I need to work out average pressure in order to work out horizontal thrust on an oil tank. I know that pressure=density*height*acceleration, although I want to know what 'top' and 'bottom' surfaces refer to. There is a tank filled with liquid, to a given depth and the height of the tank is given. So how do I work out the average pressure? Do I just sub in the to heights? The depth of the liquid and the height of the tank?