According to the NSSL (The National Severe Storms Laboratory) on the NOAA web site:
"How many inches of snow equals one inch of rain?
On average, thirteen inches of snow equals one inch of rain in the US, although this ratio can vary from two inches for sleet to nearly fifty inches for very dry, powdery snow under certain conditions."
I personally like the 10 to 1 ratio as it is easy to calculate.