  We present empirical relations for determining the amount by which the
effective temperatures and radii -- and therefore the estimated masses -- of
low-mass stars and brown dwarfs are altered due to chromospheric activity. We
base our relations on a large set of low-mass stars in the field with Halpha
activity measurements, and on a set of low-mass eclipsing binaries with X-ray
activity measurements from which we indirectly infer the Halpha activity. Both
samples yield consistent relations linking the amount by which an active
object's temperature is suppressed, and its radius inflated, to the strength of
its Halpha emission. These relations are found to approximately preserve
bolometric luminosity. We apply these relations to the peculiar brown-dwarf
eclipsing binary 2M0535-05, in which the active, higher-mass brown dwarf has a
cooler temperature than its inactive, lower-mass companion. The relations
correctly reproduce the observed temperatures and radii of 2M0535-05 after
accounting for the Halpha emission; 2M0535-05 would be in precise agreement
with theoretical isochrones were it inactive. The relations that we present are
applicable to brown dwarfs and low-mass stars with masses below 0.8 Msun and
for which the activity, as measured by the fractional Halpha luminosity, is in
the range -4.6 < log LHa/Lbol < -3.3. We expect these relations to be most
useful for correcting radius and mass estimates of low-mass stars and brown
dwarfs over their active lifetimes (few Gyr) and when the ages or distances
(and therefore luminosities) are unknown. We also discuss the implications of
this work for improved determinations of young cluster initial mass functions.
