I'll be using a mass balance calc for a tank design at work. We are designing a tank that has an automatic float switch on it, attached to a valve which will start filling the tank when the water level drops below a certain point. This tank is intended to drain quickly (flow rate not known at this time), but the fill rate will not be as large as the discharge rate. I want to have a representative model of the tank so we can ensure it will function properly. Anyway, I just want to double check to see if I did this correctly. I think I did, as its relatively simple, but another set of eyes never hurts. What I have done so far: I'll skip the intro equation sorting, and get to the generic equation below: A(dh/dt) = Qin - Qout Where, A = Tank area h = tank height Q = flow rate My attempt: I moved "A" and "dt" to the right-hand side, and got: dh = (Qin - Qout)/A * dt I then integrated to get h(t): h(t) = [(Qin-Qout)/A] * Δt I think this would be correct, as the units leave me with "ft" Q (ft3/s) * 1/A (1/ft2) * Δt (s) = ft.