The source of the gravitational-wave (GW) signal GW170817, very likely a binary neutron star merger, was alsoobserved electromagnetically, providing the first multi-messenger observations of this type. The two-week-longelectromagnetic (EM) counterpart had a signature indicative of an r-process-induced optical transient known as akilonova. This Letter examines how the mass of the dynamical ejecta can be estimated without a directelectromagnetic observation of the kilonova, using GW measurements and a phenomenological model calibrated tonumerical simulations of mergers with dynamical ejecta. Specifically, we apply the model to the binary massesinferred from the GW measurements, and use the resulting mass of the dynamical ejecta to estimate its contribution(without the effects of wind ejecta) to the corresponding kilonova light curves from various models. Thedistributions of dynamical ejecta mass range between = - - -Mej 10 10 Me3 2 for various equations of state,assuming that the neutron stars are rotating slowly. In addition, we use our estimates of the dynamical ejecta massand the neutron star merger rates inferred from GW170817 to constrain the contribution of events like this to ther-process element abundance in the Galaxy when ejecta mass from post-merger winds is neglected. We find that if 10% of the matter dynamically ejected from binary neutron star (BNS) mergers is converted to r-processelements, GW170817-like BNS mergers could fully account for the amount of r-process material observed in theMilky Way.