Generally the gas metallicity in distant galaxies can only be inferred by using a few prominent emission lines. Various theoretical models have been used to predict the relationship between emission line fluxes and metallicity, suggesting that some line ratios can be used as diagnostics of the gas metallicity in galaxies. However, accurate empirical calibrations of these emission line flux ratios from real galaxy spectra spanning a wide metallicity range are still lacking. In this paper we provide such empirical calibrations by using the combination of two sets of spectroscopic data: one consisting of low-metallicity galaxies with a measurement of [O III]{lambda}4363 taken from the literature, including spectra from the Sloan Digital Sky Survey (SDSS), and the other one consisting of galaxies in the SDSS database whose gas metallicity has been determined from various strong emission lines in their spectra. This combined data set constitutes the largest sample of galaxies with information on the gas metallicity available so far and spanning the widest metallicity range. By using these data we obtain accurate empirical relations between gas metallicity and several emission line diagnostics, including the R23 parameter, the [NII]{lambda}6584/H{alpha} and [O III]{lambda}5007/[NII]{lambda}6584 ratios.