A numerical model of the radiation belt electron distribution as a function of equatorial pitch angle, longitude, and time is combined with data from a low-altitude satellite to estimate the source and loss rates of ~0.5 to 3 MeV electrons during magnetic storms. The relative intensity levels in the stable trapping and drift loss cone regions provide a measure of the loss rate and associated pitch angle diffusion coefficient. The source rate is that required to maintain the observed stably trapped component. The source mechanism is unspecified and may include a combination of radial diffusion and local acceleration. Adiabatic effects are treated separately and are found to have a minimal influence on parameter estimation in the presence of pitch angle diffusion. Two sample storm periods provide examples of the varying source and loss rates estimated with this method. They show that significant losses during the storm main phase deplete the prestorm radiation belt on a timescale ~1 hour. It is replenished by a source that acts concurrently or at somewhat later times. Significant but variable source and loss processes may continue during the storm recovery phase. The storm time source and loss rates may result in a net increase or decrease in the stably trapped population relative to the prestorm level. These conclusions are supported by source and loss indices that are defined to provide rough estimates of the fractional source and loss rates from several additional magnetic storms and are readily calculated from the data alone.