  We selected a sample of 28 Type 1 AGNs for which a black hole mass has been
inferred using the reverberation mapping technique and single epoch scaling
relations. All 28 sources show clear evidence of the "Big Blue Bump" in the
optical-UV band whose emission is produced by an accretion disk (AD) around a
supermassive black hole. We fitted the spectrum of these sources with the
relativistic thin AD model KERRBB in order to infer the black hole masses and
compared them with those from Reverberation mapping and Single epoch methods,
discussing the possible uncertainties linked to such a model by quantifying
their weight on our results. We find that for the majority of the sources,
KERRBB is a good description of the AD emission for a wide wavelength range.
The overall uncertainty on the black hole mass estimated through the disk
fitting procedure is $\sim 0.45$ dex (which includes the uncertainty on fitting
parameters such as e.g. spin and viewing angle), comparable to the systematic
uncertainty of reverberation mapping and single epoch methods; however, such an
uncertainty can be $\leq 0.3$ dex if one of the parameters of the fit is well
constrained. Although all of the estimates are affected by large uncertainties,
the masses inferred using the three methods are compatible if the dimensionless
scale factor $f$ (linked to the unknown kinematics and geometry of the Broad
Line Region) is assumed to be larger than one. For the majority of the sources,
the comparison between the results coming from the three methods favors small
spin values. To check the goodness of the KERRBB results, we compared them with
those inferred with other models, such as AGNSED, a model that also accounts
for the emission originating from an X-ray corona: using two sources with a
good data coverage in the X band, we find that the masses estimated with the
two models differ at most by a factor of $\sim 0.2$ dex.
