A device is described for measuring the time dependent, volume averaged density of a decaying plasma in a large multimode microwave cavity such as is typically used for plasma confinement studies. The technique consists of filling the cavity with microwave radiation and digitally counting the modes observed with a detector as the plasma decays. Since each mode corresponds statistically to a constant density increment, the density at a given time is proportional to the number of modes that appear as the density decays to zero. Experiments in a toroidal octopole show good agreement with density as measured by other methods.