We present a new technique for the evaluation of hydrodynamic densities (for example mass, energy or momentum densities) at planes in molecular dynamics simulations. This technique employs an easily computed expression for the density at a plane that is formally exact, unlike other expressions such as histogram approximations. We present simple examples of applications of this procedure to the calculation of mass and momentum densities, and hence the streaming velocity, at planes in a fluid undergoing planar Poiseuille flow, and show how the temperature profile can be obtained by the same procedure.