Rain to Snow Calculator
Enter a rainfall amount and air temperature and this tool instantly shows how deep the equivalent snowfall would be, using the National Weather Service snow-to-liquid ratio table. Switch to metric or imperial units, flip to snow-to-rain reverse mode, or override the ratio with your own value. The result updates as you type, with full worked steps and a chart showing how snowfall depth changes across the temperature spectrum.
How the rain-to-snow conversion works
Snowfall depth and liquid precipitation are related by the snow-to-liquid ratio (SLR), sometimes called the snow ratio. If the SLR is 10:1, one inch of liquid water produces ten inches of snow. The ratio varies because colder air produces smaller, less dense ice crystals that trap more air. Near the freezing point (around 32 F), snow crystals are large and wet, producing a low ratio of roughly 10:1. As temperatures fall toward 10-15 F, crystals become lighter and the ratio climbs to 30:1. Below -20 F, extreme polar conditions can push the ratio to 100:1. The formula is straightforward: Snow depth = Rain amount x SLR, or reversed, Rain equivalent = Snow depth / SLR.
The National Weather Service SLR table
This calculator uses the snow-to-liquid ratio guidance published by the National Weather Service (NWS). The NWS Eastern Region HQ Technical Attachment 98-9 established the temperature-band table that meteorologists and emergency managers use for winter-storm planning. It groups temperatures into bands from just below freezing down to extreme arctic cold, assigning a representative SLR to each band. Most winter storms in the contiguous United States fall in the 27-34 F range (SLR 10:1) or the 20-27 F range (SLR 15:1). The 10:1 average is often cited as the "rule of thumb" because many storms land near the freezing point.
Wet snow vs. dry powder: why it matters
The SLR is not just a curiosity. It has practical consequences for road clearing, roof loads, and water-supply forecasting. Wet snow (low SLR) is dense and heavy. A 6-inch snowfall with a 10:1 ratio contains 0.6 inches of water, weighing roughly 5 lb per square foot on a flat surface. The same 6 inches at a 30:1 ratio contains only 0.2 inches of water, weighing closer to 1.5 lb/ft. Engineers sizing roof loads, water managers estimating spring runoff, and snow-removal crews sizing equipment all use the SLR (or its cousin, the snow water equivalent, SWE) to plan correctly. Light powder is also more susceptible to wind redistribution and drifting, which can create hazards even when snowfall totals seem modest.
Reverse mode: snow depth to liquid equivalent
The reverse calculation converts a measured or forecast snow depth back into a liquid water equivalent. This is especially useful for hydrology and water-resource management. Snowpack stores water that releases slowly as temperatures rise in spring. By dividing the snow depth by the applicable SLR (or measuring SWE directly with a snow tube or automated sensor), forecasters can estimate how much water will enter streams and reservoirs during the melt season. The same arithmetic applies to fresh snowfall: a news report of 12 inches of snow at 15 F implies about 0.4 inches of liquid equivalent, useful for comparing storm severity across regions with very different climates.
NWS snow-to-liquid ratio (SLR) by temperature
| Temperature (°F) | Temperature (°C) | SLR | Snow type |
|---|---|---|---|
| Above 34 | Above 1 | 0 (rain) | Rain / sleet mix |
| 27-34 | -3 to 1 | 10:1 | Wet, heavy snow |
| 20-27 | -6 to -3 | 15:1 | Moderate snow |
| 15-20 | -9 to -6 | 20:1 | Standard fluffy snow |
| 10-15 | -12 to -9 | 30:1 | Dry, light snow |
| 0-10 | -18 to -12 | 40:1 | Very dry powder |
| -20 to 0 | -29 to -18 | 50:1 | Arctic powder |
| Below -20 | Below -29 | 100:1 | Extreme polar powder |
Based on the National Weather Service Eastern Region HQ snow-to-liquid ratio table. Actual ratios vary by storm dynamics, crystal type, and humidity.
Frequently asked questions
How many inches of snow equals 1 inch of rain?
On average, about 10 inches of snow equals 1 inch of rain - the classic "10 to 1" rule of thumb used by the National Weather Service. However, the actual ratio depends on air temperature. Near freezing (27-34 F), heavy wet snow forms at roughly 10:1. At 15-20 F the ratio rises to 20:1, meaning one inch of rain produces 20 inches of dry, fluffy snow. At extreme cold below -20 F, the ratio can reach 100:1.
Why does colder air produce more snow from the same rainfall?
Cold air produces smaller, lighter ice crystals that have a more complex, branching structure. These crystals trap more air between them as they pile up, resulting in a lower-density snowpack. Warmer air near the melting point produces larger, rounder, wetter crystals that pack together more efficiently, forming denser snow with a lower depth-to-liquid ratio.
What is snow water equivalent (SWE)?
Snow water equivalent is the depth of liquid water that would result from melting a given snow sample. It equals the snow depth divided by the SLR. For example, 20 inches of snow with a 20:1 ratio has an SWE of 1 inch. SWE is the standard metric in hydrology and water-supply forecasting because it measures the actual water stored in the snowpack regardless of how fluffy or compact the snow is.
Can I use this calculator for metric units?
Yes. Switch the unit selector to Metric (mm, degrees Celsius) and enter rainfall or snow depth in millimeters. The temperature input automatically converts to Fahrenheit internally to look up the NWS ratio, and the results are reported back in millimeters. The SLR itself is dimensionless, so the ratio applies equally in metric and imperial measurements.
When should I use a custom snow-to-liquid ratio?
Use a custom ratio when you have an observed or forecast SLR from your local weather service, a verified historical record for your site, or a snowpack measurement from a snow tube. A custom value overrides the temperature-based lookup and is useful for unusual storm types, very humid or very dry climates, or when assessing an already-fallen snowpack whose density you have measured directly.
Does the calculator account for wind or humidity?
Not directly. The NWS temperature table provides a representative SLR for each temperature band, but the actual ratio for a specific storm also depends on wind speed, relative humidity, cloud-base height, and the type of ice crystal forming at altitude. For planning purposes the NWS table is considered a reliable standard estimate. For operational meteorology or precision hydrology, use local observations and the custom-ratio option.