We combine new data from the main sequence (M_* versus SFR) of star-forming galaxies and galaxy colors (from GALEX to Spitzer) with a flexible stellar population scheme to deduce the mass-to-light ratio (Upsilon_*) of star-forming galaxies from the SPARC and S^4G samples. We find that the main sequence for galaxies, particular the low-mass end, combined with the locus of galaxy colors, constrains the possible star formation histories of disk and dwarf galaxies to a similar shape found by Speagle et al. (2014). Combining the deduced star formation history with stellar population models in the literature produces reliable Upsilon_* values as a function of galaxy color with an uncertainty of only 0.05 dex. We provide prescriptions to deduce Upsilon_* for optical and near-IR bandpasses, with near-IR bandpasses having the least uncertainty (Upsilon_* from 0.40 to 0.55). We also provide the community with a webtool, with flexible stellar population parameters, to generate their own Upsilon_* values over the wavelength range for most galaxy surveys.
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