A theory for a nanometer scale pump based on the ratchet concept is presented. A lattice gas model with a set of moves that satisfy hydrodynamic equations is used to describe an asymmetric nanometer channel connecting two reservoirs of fluid. The channel, which is coupled to an external oscillatory (or stochastic) driving force, pumps fluid from one reservoir to the other. The frequency of the external driving force, the fluid density, and the channel dimensions are used to control the fluid flow. We observe a non-monotonic behavior of the flow with respect to some model parameters and discuss the efficiency of the device.