  This work focuses on active galactic nuclei (AGNs), and the relation between
the sizes of the hot dust continuum and the broad-line region (BLR). We find
that the continuum size measured using optical/near-infrared interferometry
(OI) is roughly twice that measured by reverberation mapping (RM). Both OI and
RM continuum sizes show a tight relation with the H$\beta$ BLR size with only
an intrinsic scatter of 0.25 dex. The masses of supermassive black holes (BHs)
can hence be simply derived from a dust size in combination with a broad line
width and virial factor. Since the primary uncertainty of these BH masses comes
from the virial factor, the accuracy of the continuum-based BH masses is close
to those based on the RM measurement of the broad emission line. Moreover, the
necessary continuum measurements can be obtained on a much shorter timescale
than those required monitoring for RM, and are also more time efficient than
those needed to resolve the BLR with OI. The primary goal of this work is to
demonstrate measuring the BH mass based on the dust continuum size with our
first calibration of the $R_\mathrm{BLR}$-$R_\mathrm{d}$ relation. The current
limitation and caveats are discussed in detail. Future GRAVITY observations are
expected to improve the continuum-based method and have the potential to
measure BH masses for a large sample of AGNs in the low-redshift Universe.
