Snowfall Accumulation Calculator
Estimate snowfall accumulation from precipitation amounts and air temperature.
Convert snow depth to snow water equivalent using typical snow density ratios.
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 (10:1) — 10 inches of snow = 1 inch of water. But this varies significantly:
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:
| Snow Type | Liquid Equivalent Ratio | Density (g/cm³) |
|---|---|---|
| Very dry powder | 3–5% (20:1 to 33:1) | 0.03–0.05 |
| Dry snow | 5–8% (12:1 to 20:1) | 0.05–0.08 |
| Average snow | 10–12% (8:1 to 10:1) | 0.10–0.12 |
| Wet snow | 12–20% (5:1 to 8:1) | 0.12–0.20 |
| Very wet/compacted | 20–33% (3:1 to 5:1) | 0.20–0.33 |
| Ice | 100% (1:1) | 0.90 |
Worked example: 24 inches of snow fell overnight. Temperature was −3°C (wet, heavy snow), density ratio estimated at 15% (6.7:1).
- SWE = 24 × 0.15 = 3.6 inches of water equivalent
- Roof load = 3.6 × 5.2 = 18.7 lbs/sq ft
- A 2,000 sq ft roof carries: 18.7 × 2,000 = 37,400 lbs (18.7 tons)
- Residential roofs are typically rated 20–40 lbs/sq ft: this storm is at the edge of capacity for minimal-construction roofs.
If two more storms add 12 inches each, the cumulative load could exceed structural limits — a clear case for roof raking or snow removal.