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Snowfall Calculator
Convert snowfall depth to water equivalent (SWE), calculate the weight of snow on your roof, and estimate snow water content. Essential for winter storm preparation, snow removal planning, and understanding snowfall water value for drought monitoring.
Not all snow is equal — fresh fluffy snow can be mostly air while wet heavy snow contains much more water. The snow-water equivalent (SWE) tells you how much liquid water is contained in a snowpack. This matters for water resource management, flood forecasting, and critically for assessing roof snow loads.
Formula
Snow Water Equivalent (inches) = Snow Depth (inches) / Snow Ratio | Roof Load (psf) = Snow Density (lb/ft³) × Depth (ft)
Snow Types and Water Content
The snow ratio (snow depth to water equivalent) varies dramatically based on temperature and humidity when snow forms. Light fluffy powder at very cold temperatures has a ratio of 30:1 (30 inches of snow = 1 inch of water). Wet heavy snow near freezing has a ratio of 8:1 or lower — nearly as dense as water.
Snow Type
Temperature
Snow Ratio
Weight per Inch Depth
Dry Powder
<15°F (-9°C)
25:1 – 30:1
~3-4 lbs/ft²/inch
Normal Snow
15-25°F (-9 to -4°C)
12:1 – 15:1
~5-6 lbs/ft²/inch
Average Snow
25-32°F (-4 to 0°C)
8:1 – 12:1
~7-9 lbs/ft²/inch
Wet Snow
Near freezing
5:1 – 8:1
~10-13 lbs/ft²/inch
Sleet/Ice
Near 32°F (0°C)
1:1 – 3:1
~15-20 lbs/ft²/inch
Roof Snow Load Safety
Most residential roofs are designed to support 20-40 lbs per square foot (psf) of snow load depending on geographic location (higher in heavy snow regions). Fresh snow weighs approximately 5-20 lbs per cubic foot. After settling and metamorphism: 15-20 lbs/ft³. Wet snow: 20-30 lbs/ft³. Ice: 57 lbs/ft³. Warning signs of dangerous roof load: creaking sounds, jammed doors, visible sagging, or cracked walls near the roofline.
Snow Water Equivalent for Agriculture
Snowpack is critically important for western US water supply — snowmelt provides 70% of freshwater in many western states. The USDA Natural Resources Conservation Service (NRCS) monitors SWE at hundreds of SNOTEL stations. A 120% of average SWE by April 1 predicts good irrigation water availability; below 80% suggests drought conditions for the following summer.
Frequently Asked Questions
Snow water equivalent (SWE) is the depth of liquid water that would result if the snow melted. It is the most important measurement in snowpack monitoring. A 12-inch snowpack at 10:1 ratio has 1.2 inches SWE. A 12-inch wet snowpack at 8:1 has 1.5 inches SWE — 25% more water despite the same depth.
Fresh light snow: 3-5 lbs per square foot per foot of depth. Average snow: 6-8 lbs/ft²/ft. Wet snow: 12-15 lbs/ft²/ft. Old settled snow: 15-20 lbs/ft²/ft. Ice: 57 lbs/ft²/ft. A 12-inch layer of average snow weighs 6-8 lbs/sq ft — a 1,000 sq ft roof carries 6,000-8,000 lbs of snow.
Most US residential roofs are designed for 20-40 psf snow load. A 2-foot layer of average snow (~12 psf/foot) = 24 psf — near typical design loads. Add ice: 1 inch of ice = 5 psf. Total 25-30 psf is approaching safe limits for some roofs. Signs of dangerous load: cracking, creaking, stuck doors, visible deflection.
Divide snow depth by the snow ratio. For average snow (10:1 ratio): 10 inches of snow = 1 inch of rain equivalent. For wet snow (8:1): 8 inches = 1 inch rain. For dry powder (20:1): 20 inches = 1 inch rain. The NRCS uses 10:1 as the standard ratio when specific data is not available.
At average 10:1 ratio: 12 inches of snow = 1.2 inches of water. For a 1,000 sq ft area: 1.2 inches water × 1,000 sq ft × 0.623 = 748 gallons of water. Western mountain snowpack representing 12 feet (144 inches) at 10:1 = 14.4 inches SWE — thousands of gallons per acre of watershed.
Wet snow (near 32°F) is much denser than dry powder. Wet snow can weigh 12-20 lbs per cubic foot vs 3-5 lbs for dry powder. A tree branch 2 inches wide and 6 feet long collecting 2 inches of wet snow might carry 15-20 lbs of snow weight — sufficient to break typical tree branches. Wet snow is also more adhesive, accumulating rather than sliding off.