  We present empirical relations between the global dust reddening and other
physical galaxy properties including the Halpha luminosity, Halpha 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 (SDSS). The reddening parameterized
by color excess E(B-V) is derived from the Balmer decrement. Besides the
dependency of reddening on Halpha 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 give
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 relation can be reproduced if most dust
attenuation to the HII region is due to diffuse interstellar dust distributing
in a disc thicker than that of HII regions. The empirical formulae can be
incorporated into semi-analytical models of galaxy formation and evolution to
estimate the dust reddening and enable comparison with observations more
practically.
