  A fast and practical method is described for calculating the microlensed flux
variance of an arbitrary source by uncorrelated stars. The required inputs are
the mean convergence and shear due to the smoothed potential of the lensing
galaxy, the stellar mass function, and the absolute square of the Fourier
transform of the surface brightness in the source plane. The mathematical
approach follows previous authors but has been generalized, streamlined, and
implemented in publicly available code. Examples of its application are given
for Dexter and Agol's inhomogeneous-disk models as well as the usual gaussian
sources. Since the quantity calculated is a second moment of the magnification,
it is only logarithmically sensitive to the sizes of very compact sources.
However, for the inferred sizes of actual QSOs, it has some discriminatory
power and may lend itself to simple statistical tests. At the very least, it
should be useful for testing the convergence of microlensing simulations.
