The calculated data is based on information from the National Weather Service that indicates the average snowfall is in a ratio of 10 inches of snow to 1 inch of equivalent rainfall. More densely packed and wet snow is in the ratio of 8 inches of snow to 1 inch of equivalent rainfall, and more powdery snow is about 14 to 1 ratio. Obviously this is an approximation but a good one. The NWS uses calculations each year in specific areas where the density ratio may be slightly different, to approximate how much water will be returned to the natural rivers, lakes, ponds, catch basins, waterways and underground streams for many different uses in later times of need. It can also be used in the calculation of how much water should be or could be released from dams for irrigation at a specific time.
It is not important what the unit of measure is, inches, feet or metric values. The ratio remains the same.