In this paper we present the updated empirical radio surface- brightness-to-diameter (?-D) relation for Galactic supernova remnants (SNRs) calibrated using 110 SNRs with reliable distances. We apply orthogonal fitting procedure and kernel density smoothing in the ? ? D plane and compare the results with the latest theoretical ? - D relations derived from simulations of radio evolution of SNRs. We argue that the best agreement between the empirical and simulated ? ? D relations is achieved if the mixed-morphology SNRs and SNRs of both, low brightness and small diameter, are filtered out from the calibration sample. The distances to 5 newly discovered remnants and 27 new candidates for shell SNRs are estimated from our full and filtered calibration samples.