We present empirical relations between the global dust reddening and other physical galaxy properties including the Hα luminosity, Hα surface brightness, metallicity and axial ratio for star-forming disc galaxies. The study is based on a large sample of ∼22 000 well-defined star-forming galaxies selected from the Sloan Digital Sky Survey. The reddening parametrized by colour excess E(B−V) is derived from the Balmer decrement. Besides the dependency of reddening on Hα luminosity/surface brightness and gas phase metallicity, it is also correlated with the galaxy inclination, in the sense that edge-on galaxies are more attenuated than face-on galaxies at a given intrinsic luminosity. In light of these correlations, we present the empirical formulae of E(B−V) as a function of these galaxy properties, with a scatter of only 0.07 mag. The empirical relations can be reproduced if most dust attenuation to the H ii region is due to diffuse interstellar dust distribution in a disc thicker than that of the H ii region. The empirical formulae can be incorporated into semi-analytical models of galaxy formation and evolution to estimate the dust reddening and more practically enable comparison with observations.