Rain to Snow Calculator
Estimate how much snow a given amount of liquid precipitation would make. Enter rain in inches or millimeters and the expected surface temperature in °F or °C — or pick the snow-to-liquid ratio yourself — and get snow depth in inches and centimeters.
Example: with Liquid precipitation 1 · Precipitation unit inches · Surface temperature during the storm 30 · Temperature unit °F · Snow-to-liquid ratio Auto — from temperature → Estimated snowfall: 10 in of snow (25.4 cm).
- Ratio used10 : 1 snow-to-liquid ratio (28–34°F: near freezing, average density)
- What that snow is likeClassic snowball-packing snow, about 10% water by volume
Computed by the calculator below using its default values. Change any input to see your own numbers.
Snow = liquid × snow-to-liquid ratio. The classic average is 10:1 near freezing; cold storms run 15:1 to 50:1, warm slush closer to 5:1.
Where the 10:1 rule comes from
The 10-to-1 snow-to-liquid ratio dates to 19th-century weather observing: melt down fresh snow near freezing and you typically recover about a tenth of its depth as water. The National Weather Service still uses 10:1 as the default first guess, which is why "an inch of rain equals ten inches of snow" is the number everyone knows.
It is an average, not a law. Measured storm ratios in the US routinely span 3:1 to 40:1 and beyond. Temperature is the biggest single lever, which is what the auto mode uses — the commonly published NWS-style bands running from 5:1 slush near 35°F to 50:1 fluff below zero.
Why cold snow is fluffier
Snow crystals grow their classic wide dendrite arms when the cloud layer sits around −12 to −18°C, and dry, cold flakes stack with lots of trapped air. Near freezing, flakes partially melt, rime, and compact — less air, denser snow. The water content is identical either way: an inch of 5:1 slush holds the same water as ten inches of 50:1 powder, which is exactly why the wet stuff is so much heavier to shovel and harder on roofs and power lines.
How it’s calculated
Snow depth = liquid precipitation × snow-to-liquid ratio (SLR). Auto mode assigns the ratio from surface temperature using commonly published NWS-style climatology bands: 40°F+ rain (0), 35–39°F 5:1, 28–34°F 10:1, 20–27°F 15:1, 15–19°F 20:1, 10–14°F 30:1, 0–9°F 40:1, below 0°F 50:1. Conversions: mm ÷ 25.4 = inches; snow cm = inches × 2.54; °F = °C × 9/5 + 32.
Real SLR depends on the whole atmospheric column (dendrite growth zone, riming, wind compaction), not just surface temperature — actual storm ratios routinely differ from these climatological bands.
Snow-to-liquid ratio by temperature
| Surface temp (°F) | Ratio | Snow from 1 in of liquid |
|---|---|---|
| 40°F and up | rain | 0 in |
| 35–39°F | 5:1 | 5 in of slush |
| 28–34°F | 10:1 | 10 in |
| 20–27°F | 15:1 | 15 in |
| 15–19°F | 20:1 | 20 in |
| 10–14°F | 30:1 | 30 in |
| 0–9°F | 40:1 | 40 in |
| Below 0°F | 50:1 | 50 in |
Commonly published NWS-style snow-ratio climatology bands; actual storm ratios vary with the full temperature profile aloft.
Common mistakes
- Applying 10:1 to a 15°F storm — powder ratios of 20:1 or more can double the depth the rule of thumb predicts.
- Using the forecast daytime high instead of the temperature expected while the precipitation is actually falling.
- Converting millimeters of rain straight to inches of snow without dividing by 25.4 first.
- Treating the output as a forecast — SLR shifts within a single storm as warm and cold layers move through.
Frequently asked questions
How much snow would 1 inch of rain be?
About 10 inches of snow near freezing (the 10:1 rule), but anywhere from 5 inches of slush in the mid-30s°F to 40–50 inches of ultra-light powder in subzero cold.
What is the formula?
Snow depth = liquid precipitation × snow-to-liquid ratio. The ratio comes from temperature: 10:1 at 28–34°F, 15:1 at 20–27°F, 20:1 at 15–19°F, and higher as it gets colder.
What is the 10:1 rule?
The long-standing observing convention that fresh snow near freezing contains about 10% water — so 1 inch of melted precipitation equals roughly 10 inches of snow. It is the NWS default assumption, not a physical constant.
Why does temperature change the ratio so much?
Cold air grows dry, branching dendrite crystals that stack with lots of trapped air; near-freezing flakes partially melt and compact. Same water, very different depth.
Is this how forecasters predict snowfall?
Modern forecasts model the ratio from the entire atmospheric column — the dendrite growth zone aloft matters more than surface temperature. This page is the transparent rule-of-thumb version of that calculation.