We use a sample of 800 galaxies with H i mass measurements from the HyperLeda catalogue and optical photometry from the fourth data release of the Sloan Digital Sky Survey (SDSS) to calibrate a new photometric estimator of the H i-to-stellar-mass ratio for nearby galaxies. Our estimator, which is motivated by the Kennicutt–Schmidt star formation law, is log₁₀(G_{H i}/S) =−1.732 38(g−r) + 0.215 182μ_i− 4.084 51, where μ_i is the i-band surface brightness and g−r is the optical colour estimated from the g- and r-band Petrosian apparent magnitudes. This estimator has a scatter of σ= 0.31 dex in log (G_{H i}/S), compared to σ∼ 0.4 dex for previous estimators that were based on colour alone. We investigate whether the residuals in our estimate of log (G_{H i}/S) depend in a systematic way on a variety of different galaxy properties. We find no effect as a function of stellar mass or 4000 Å break strength, but there is a systematic effect as a function of the concentration index of the light. We then apply our estimator to a sample of 10⁵ emission-line galaxies in the SDSS Data Release 4 (DR4) and derive an estimate of the H i mass function, which is in excellent agreement with recent results from H i blind surveys. Finally, we re-examine the well-known relation between gas-phase metallicity and stellar mass, and ask whether there is a dependence on H i-to-stellar-mass ratio, as predicted by chemical evolution models. We do find that gas-poor galaxies are more metal rich at fixed stellar mass. We compare our results with the semi-analytic models of De Lucia & Blaizot, which include supernova feedback, as well as the cosmological infall of gas.