Depending on which source you use, the conversion of snow inches to rain inches varies. Many factors can influence the conversion, like the crystal structure and moisture content of the snow. (Some sources say 10" of snow = 1" of rain). The conditions of temperature and humidity in the cloud determine the type of snow crystals that form. At close to 5 degrees F, beautiful, starlike crystals form. These crystals, called "dendrites" because they have many delicate branches, make the fluffiest snow.
An easy way - and mostly accurate - for the homeowner is to assume 1 inch of rain is equal to 1 foot of snow, a 12-1 ratio. Depending on the temperature and moisture profiles of the snow growth region of the atmosphere and the origin area of the storm system, that ratio can go higher, say 20-1, which would be 20 inches, or lower, say 6-1 or so.
12-1 is most forecasters starting point, and if you go to your local NWS page and read the area forecast discussion, they usually tell what ratio they believe a system will have as it moves through your area.