Rain to Snow Calculator (liquid precipitation → snow depth by snow-to-liquid ratio)

Understanding the rain to snow calculator — concept and principle

This calculator converts a liquid precipitation amount into an expected depth of new snow using a snow-to-liquid ratio. It also shows the depth at the US mean ratio and at the typical low and high ratios, the new snow's density and the liquid equivalent. Weather watchers, forecasters and road crews use it.

The same water makes deeper snow when flakes are fluffy; the ratio, not the rain, is the uncertain part.

How it is calculated

Snow depth = liquid equivalent × snow-to-liquid ratio (SLR)

Density of new snow = 1,000 kg/m³ ÷ SLR

Liquid amount in mm or inches; depth shown in cm and inches.
Reference ratios from a US climatology: mean 13.5, quartiles 9.3 and 16.7.

The principle

Snowflakes trap a lot of air, so the same amount of water makes a much deeper layer as snow than as rain. The snow-to-liquid ratio says how many times deeper. The traditional rule of thumb is 10:1, but real ratios vary widely: cold, dry snow made of light dendrites has much higher ratios, while wet snow near freezing has much lower ones. A US study of observations from 1971–2000 found a mean of 13.5, a median of 12.1 and a most common value of 10.

Worked example

Default inputs: 10 mm liquid, ratio 10:1.

Snow depth = 10 mm × 10 = 100 mm = 10 cm = 3.9 in.
At the US mean ratio 13.5:1: 13.5 cm.
Low estimate (9.3:1): 9.3 cm; high estimate (16.7:1): 16.7 cm.
Density of new snow = 1,000 ÷ 10 = 100 kg/m³; liquid equivalent 10 mm.

Each point is the calculator re-run with only “Liquid precipitation (rain equivalent)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Liquid precipitation (rain equivalent)	Snow depth (cm)
2.5	2.5
5	5
7.5	7.5
10	10
12.5	12.5
15	15
20	20

How to read the result

Half of observed cases in the US study fell between 9.3:1 and 16.7:1, so the low and high figures give a realistic spread. Colder air usually means fluffier snow and higher ratios. Snow also settles after it falls, so measured depth later in the day is lower.

Common mistakes

Treating 10:1 as a fixed rule.
Ignoring melting on warm ground.
Confusing new-snow depth with total snow depth on the ground.

Keep in mind: Snow ratios vary with temperature, wind and crystal type. Use official forecasts for travel and safety decisions.

Frequently asked questions

How much snow does 1 inch of rain make?

About 10 inches at 10:1, but often 9 to 17 inches.

Why is cold snow fluffier?

Cold, dry conditions favour light, branching crystals that trap more air.

What is the density of fresh snow?

At 10:1, about 100 kg/m³, one tenth of water.

Does wind change the ratio?

Yes; wind breaks crystals and packs snow, lowering the ratio.

Sources: M. A. Baxter, C. E. Graves and J. T. Moore, "A climatology of snow-to-liquid ratio for the contiguous United States", Weather and Forecasting 20, 729–744 (2005)

Checked 9 Nov 2026. Baxter, M. A., Graves, C. E. & Moore, J. T. (2005) “A climatology of snow-to-liquid ratio for the contiguous United States”, Weather and Forecasting 20: 729–744 (mean 13.53, median 12.14, mode 10.0, quartiles 9.26 and 16.67).

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