  We present a methodology for the determination of empirical masses of single
stars through the combination of three direct observables with Gaia and TESS:
(i) the surface gravity via granulation-driven variations in the TESS light
curve, (ii) the bolometric flux at Earth via the broadband spectral energy
distribution, and (iii) the distance via the Gaia parallax. We demonstrate the
method using 525 Kepler stars for which these measures are available in the
literature, and show that the stellar masses can be measured with this method
to a precision of $\sim$25\%, limited by the surface-gravity precision of the
granulation "flicker" method ($\sim$0.1~dex) and by the parallax uncertainties
($\sim$10\% for the Kepler sample). We explore the impact of expected
improvements in the surface gravity determinations---through the application of
granulation background fitting and the use of recently published
granulation-metallicity relations---and improvements in the parallaxes with the
arrival of the Gaia second data release. We show that the application of this
methodology to stars that will be observed by TESS should yield radii good to a
few percent and masses good to $\approx$10\%. Importantly, the method does not
require the presence of an orbiting, eclipsing, or transiting body, nor does it
require spatial resolution of the stellar surface. Thus we can anticipate the
determination of fundamental, accurate stellar radii and masses for hundreds of
thousands of bright single stars---across the entire sky and spanning the
Hertzsprung-Russell diagram---including those that will ultimately be found to
host planets.
