Abstract Conventional estimator of rainfall rate using reflectivity factor and differential reflectivity ZDR becomes unstable when the measured values of ZDR are small due to measurement errors. An alternate estimator of rainfall rate using reflectivity factor and ZDR is derived, so that this estimator is fairly robust over the full dynamic range of reflectivity factor and ZDR. Simulations are used to study the error structure of this robust estimator in comparison with the conventional estimator of rainfall rate. It is shown that the alternate estimator performs better than the conventional estimator of rainfall rate at all rainfall values. In particular the largest improvement of this estimator is proved to be in light rain. The robust estimator is obtained as a direct regression of rainfall rate against reflectivity factor and ZDR instead of solving for the drop size distribution.