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Snowfall Water Equivalent Calculator

Reviewed by the CalculatorHub team, edited by James Graham. NRCS/NOAA methodology for SWE estimation. Last verified 14 June 2026.

Snow water equivalent (SWE) is the depth of liquid water that would result if a given depth of snow were completely melted. It is the standard metric for quantifying water stored in snowpacks and is used by water managers, flood forecasters and engineers across the United States. This calculator uses the fundamental SWE formula: SWE equals snow depth multiplied by the snow density ratio. Select a common snow type for a preset density ratio, or enter your own. Results are shown in both inches and millimetres.

Total depth of snow on the ground or in a single storm event
Select a snow type or choose custom to enter your own ratio
1.20 in
30.48 mm

SWE formula

SWE = Snow depth × (rhosnow / rhowater)

Where rho(snow) is snow density in kg/m3 and rho(water) is water density (approximately 1,000 kg/m3). The ratio rho(snow)/rho(water) is the dimensionless density ratio. For example, fresh powder at 50 kg/m3 gives a ratio of 0.05, so 20 inches of powder equals 1 inch of SWE.

Typical snow density ranges (NRCS / NOAA)

  • Very light powder (below -15 deg C): Density ratio 0.03 to 0.05 (30:1 to 20:1 snow-to-water ratio)
  • Typical new snow: Density ratio 0.08 to 0.12 (12:1 to 8:1 ratio)
  • Settled snow: Density ratio 0.15 to 0.25
  • Wind-packed snow: Density ratio 0.25 to 0.40
  • Wet snow / slush: Density ratio 0.30 to 0.50

Snow water equivalent: frequently asked questions

What is snow water equivalent (SWE)?

Snow water equivalent (SWE) is the depth of water that would result if the snow were completely melted. It is the primary metric used by water resource managers, flood forecasters and avalanche centres to quantify the amount of water stored in a snowpack. SWE is measured in inches or millimetres and is critical for spring runoff forecasting and drought monitoring.

What is the snow water equivalent formula?

SWE = Snow depth x (snow density / water density). Snow density is expressed as a ratio of snow density to water density (approximately 1,000 kg/m3). Fresh fluffy snow has a ratio of about 0.05 (5%), meaning 20 inches of fresh snow equals about 1 inch of water. Wet heavy snow has a ratio of 0.20 to 0.35. The standard rule of thumb of 10:1 (ten inches of snow equals one inch of water) corresponds to a density ratio of 0.10.

What is the 10:1 snow-to-water ratio?

The 10:1 ratio (10 inches of snow per 1 inch of liquid water equivalent) is a widely used rule of thumb in the US for typical mid-latitude snowfall. However, the National Weather Service and NRCS note that actual ratios vary from about 5:1 for very wet heavy snow near 0 deg C to 30:1 or more for very light, cold powder snow. The density ratio of 0.10 represents the mid-range default used in NWS precipitation estimates.

How is SWE measured in the field?

The NRCS Natural Resources Conservation Service operates the SNOTEL (SNOwpack TELemetry) network, which uses automated snow pillows and snow courses to measure SWE across the western US. Manual measurement uses a snow tube (Federal sampler) pushed into the snowpack; the weight of the core divided by the tube area gives SWE directly. The USGS also maintains streamflow gauges that are used to calibrate SWE-to-runoff models.

Why does snow density vary?

Snow density varies with temperature, wind, crystal type, age and compaction. Fresh snow from very cold air (below -10 deg C) contains low-density dendritic crystals with much air trapped between them, giving densities of 30 to 50 kg/m3. Wet spring snow near 0 deg C can have densities of 300 to 500 kg/m3. Settled, old snow is typically 200 to 300 kg/m3, and wind-packed snow can exceed 400 kg/m3.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.