Snow Water Equivalent Calculator
Snow water equivalent (SWE) is the depth of liquid water that would be produced by melting a given column of snow. It is calculated as snow depth multiplied by the ratio of snow density to water density. SWE is the most important snowpack measurement for water supply forecasting because it directly represents how much water is stored in the snowpack. The USDA NRCS SNOTEL network measures SWE at over 900 automated sites across the western US. This calculator converts snow depth and density to SWE in inches and millimeters, and also computes the volume of water stored per unit area.
SWE formula
SWE (inches) = Snow depth (inches) * Snow density (%) / 100
SWE (mm) = SWE (inches) * 25.4
Snow-to-water ratio = 100 / Snow density (%)
Snow density is expressed as a percentage of liquid water density (1,000 kg/m3 or 62.4 lb/ft3). A 25 percent density means 1 foot of snow contains 0.25 feet (3 inches) of water. The snow-to-water ratio is the inverse: 100/25 = 4, meaning 4 inches of snow per 1 inch of water.
Snowpack and water resources
- Western US snowpack provides about 75 percent of water supplies in the region (NRCS).
- April 1 snowpack is the traditional benchmark date for water supply forecasting in the western US.
- Sierra Nevada snowpack in California has declined 25 percent since 1950 due to warming temperatures (USGS).
- A "snow drought" occurs when SWE is significantly below the historical median for the date, threatening summer streamflows.
- The NRCS National Water and Climate Center publishes monthly water supply outlooks and SNOTEL basin summaries.
Frequently asked questions
What is snow water equivalent (SWE)?
Snow water equivalent (SWE) is the depth of liquid water that would result from melting a given layer of snow. It equals snow depth multiplied by the ratio of snow density to water density. SWE is the key measurement for water supply forecasting in mountain watersheds.
What is typical snow density?
Fresh light snow has a density of 50 to 100 kg/m3 (5 to 10 percent of water density). Settled snow is 200 to 300 kg/m3 (20 to 30 percent). Wind-packed snow reaches 300 to 400 kg/m3. Firn (old compressed snow) is 400 to 830 kg/m3. Glacier ice is 830 to 917 kg/m3. Water density is 1,000 kg/m3.
How does NRCS measure SWE?
The USDA Natural Resources Conservation Service (NRCS) operates the SNOTEL (SNOwpack TELemetry) network with over 900 automated stations that continuously measure SWE using snow pillows (pressure sensors under the snowpack) and depth sensors. Data is available at nwcc.nrcs.usda.gov.
Why is SWE important for water supply?
In the western US, mountain snowpack stores water through winter and releases it as melt during spring and summer, supplying irrigation, municipal water, and hydropower. The NRCS Western Snowpack Report issues monthly SWE forecasts that are critical for drought planning and water allocation.
What is the standard snow density ratio for weather reporting?
The National Weather Service uses a standard ratio of 10:1 (10 inches of snow = 1 inch of water) for reporting snowfall, corresponding to a snow density of 100 kg/m3 (10%). However, actual ratios range from 3:1 for heavy wet snow to 40:1 for very light dry snow.
Official sources
- NRCS: NRCS Water and Climate Center (SNOTEL).
- NOAA: NOAA Snow Resources.
- USGS: USGS Water Science School: Snow and Ice.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.