In many applications we know snow depth, but need to compute the snow water equivalent (SWE). Measuring SWE takes ~20 times as long as measuring depth so worldwide, depth measurements outnumber measurements of SWE. Here we present a method of converting depth into SWE by estimating the bulk density of the snow. Estimates of bulk density can be made from historical data with little loss of accuracy compared to field measurements. The reliability of the predictions rests on the fact that bulk density varies over a narrow range, and that regression of SWE against depth can explain 95% of the SWE variance. Using a Bayesian analysis of 25,401 depth-density-SWE data collected in the U.S., Canada, and Switzerland, we generate statistical relationships and look-up tables that give the bulk density as a function of depth and snow class, the latter taken from published maps. Using tabulated bulk densities and measured or estimated depths, reliable depth-to-SWE conversions can be done on a site-by-site basis for modeling or global water resource assessment.