Snowfall Accumulation Calculator

Snowfall water equivalent (SWE) is the depth of water that would result if a given accumulation of snow melted completely. It is critical for hydrology, flood forecasting, water supply planning, and structural load calculations.

Snow water equivalent formula: SWE (inches) = Snow Depth (inches) × Snow Density Ratio

Snow density ratio: Ratio = Liquid Equivalent ÷ Snow Depth

Typical ratio: 0.10, written 10:1, meaning 10 inches of snow melts to 1 inch of water.

⚠️ Two conventions, and they are reciprocals. A “15% ratio” and a “15:1 ratio” are not the same thing and are not even close: 15% is 6.7:1 wet snow, while 15:1 is 6.7% dry snow. This page uses the X:1 depth form throughout, which is what forecasters use. When you see a percentage quoted elsewhere, invert it before comparing.

Weight of snow on roof: Load (lbs/sq ft) = SWE (inches) × 5.2 (Water weighs 5.2 lbs per inch-foot; approximately 5.2 lbs/sq ft per inch of SWE)

What each variable means:

• SWE: the standardized measure; used in weather reporting and hydrology calculations
• Snow density: depends on temperature and moisture content of falling snow; ranges from 3% (very dry, powder) to 33% (wet, heavy, near-freezing)
• 10:1 ratio: the standard assumption; valid for typical temperatures −5°C to −10°C
• Structural load: fresh snow at 10:1 is light; wet spring snow at 3:1 can weigh 3× as much per depth; combined with ice layers, roof collapses occur

Reference: snow density by type, and the band this calculator uses:

The temperature bands are a rule of thumb and a decent one, but they are not the whole story. Wind packs snow as it falls, so a windy 20°F storm lays down denser snow than a calm one at the same temperature, and a storm that starts as rain and turns over to snow leaves a heavy wet layer underneath a fluffy one.

Which direction this calculator runs. Forecasts give you liquid precipitation, so that is the input here and snow depth is the output. The reference table above runs the other way, from a depth you have measured back to its water content. Both are the same relationship; just be clear which number you have.

Worked example, matching the calculator: The forecast calls for 1.6 inches of liquid precipitation at 25°F.

• 25°F falls in the 15:1 band, average snow
• Snow depth = 1.6 × 15 = 24 inches
• Water equivalent stays 1.6 inches, because that is what fell
• Roof load = 1.6 × 5.2 = 8.3 lbs per sq ft
• Over a 2,000 sq ft roof: 8.3 × 2,000 = 16,600 lbs, about 8.3 tons

Two feet of snow sounds alarming and at this density it is not: 8 lbs per sq ft is well inside the 20 lbs per sq ft minimum a residential roof is built to. What changes that is rain. The same 1.6 inches of water landing as wet snow near freezing gives 16 inches of depth at nearly 3 times the density, and a warm rain falling onto snow already on the roof adds its own weight without adding any height at all. That is the situation that collapses roofs, and from the ground nothing looks different.

Use the roof snow load calculator for snow already sitting up there, where depth and density are what you measure.