  We use galaxies from the Herschel Reference Survey to evaluate commonly used
indirect predictors of cold gas masses. We calibrate predictions for cold
neutral atomic and molecular gas using infrared dust emission and gas depletion
time methods which are self-consistent and have ~20% accuracy (with the highest
accuracy in the prediction of total cold gas mass). However, modest systematic
residual dependences are found in all calibrations which depend on the
partition between molecular and atomic gas, and can over/under-predict gas
masses by up to 0.3dex. As expected, dust-based estimates are best at
predicting the total gas mass while depletion time-based estimates are only
able to predict the (star-forming) molecular gas mass. Additionally, we advise
caution when applying these predictions to high-z galaxies, as significant
(0.5dex or more) errors can arise when incorrect assumptions are made about the
dominant gas phase. Any scaling relations derived using predicted gas masses
may be more closely related to the calibrations used than to the actual
galaxies observed.
