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In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, $ϒ$ ,^[1] is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity. These ratios are often reported^[why?] using the value calculated for the Sun as a baseline ratio which is a constant $ϒ ☉$ = 5133 kg/W: equal to the solar mass M_☉ divided by the solar luminosity L_☉, ⁠M_☉/L_☉⁠. The mass-to-light ratios of galaxies and clusters are all much greater than $ϒ ☉$ due in part to the fact that most of the matter in these objects does not reside within stars and observations suggest that a large fraction is present in the form of dark matter.

Luminosities are obtained from photometric observations, correcting the observed brightness of the object for the distance dimming and extinction effects. In general, unless a complete spectrum of the radiation emitted by the object is obtained, a model must be extrapolated through either power law or blackbody fits. The luminosity thus obtained is known as the bolometric luminosity.

Masses are often calculated from the dynamics of the virialized system or from gravitational lensing. Typical mass-to-light ratios for galaxies range from 2 to 10 $ϒ ☉$ while on the largest scales, the mass to light ratio of the observable universe is approximately 100 $ϒ ☉$ , in concordance with the current best fit cosmological model.