  We present a self-consistent cross-calibration of the three main molecular
gas mass tracers in galaxies, the $\rm ^{12}CO$(1-0), [CI]($^3P_1$-$^3P_0$)
lines, and the submm dust continuum emission, using a sample of 407 galaxies,
ranging from local disks to submillimetre-selected galaxies (SMGs) up to $z
\approx 6$. A Bayesian method is used to produce galaxy-scale universal
calibrations of these molecular gas indicators, that hold over 3-4 orders of
magnitude in infrared luminosity, $L_{\rm IR}$. Regarding the dust continuum,
we use a mass-weighted dust temperature, $T_{\rm mw}$, determined using new
empirical relations between temperature and luminosity. We find the average
$L/M_{\rm mol}$ gas mass conversion factors to be $\alpha_{850}=
6.9\times10^{12}\,\rm W\,Hz^{-1}\,M_{\odot}^{-1}$, $\alpha_{\rm CO} = \rm
4\,M_{\odot} (K\,km\,s^{-1}\,pc^2)^{-1}$ and $\alpha_{\rm CI} = \rm 17.0
\,M_{\odot} (K\,km\,s^{-1}\,pc^2)^{-1}$, based on the assumption that the mean
dust properties of the sample ($\kappa_H$ = gas-to-dust ratio/dust emissivity)
will be similar to those of local metal rich galaxies and the MW. The tracer
with the least intrinsic scatter is [CI](1-0), while CO(1-0) has the highest.
The conversion factors show a weak but significant correlation with $L_{\rm
IR}$. Assuming dust properties typical of metal-rich galaxies, we infer a
neutral carbon abundance $X_{\rm CI} = [C^0/\rm mol]=1.6\times 10^{-5}$,
similar to that in the MW. We find no evidence for bimodality of $\alpha_{\rm
CO}$ between main-sequence (MS) galaxies and those with extreme star-formation
intensity, i.e. ULIRGs and SMGs. The means of the three conversion factors are
found to be similar between MS galaxies and ULIRGs/SMGs, to within 10-20%. We
show that for metal-rich galaxies, near-universal average values for
$\alpha_{\rm CO}$, $X_{\rm CI}$ and $\kappa_H$ are adequate for global
molecular gas estimates.
