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Snow Water Equivalent Calculator

Convert a depth of snow into the amount of liquid water it holds, from the snow depth and its density or snow-to-water ratio.

Snow depth

Snow density

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How it works

SWE (mm) = snow depth (mm) × snow density ÷ 1000 · Snow-to-water ratio = 1000 ÷ density

Snow water equivalent (SWE) is the depth of water you would get if a snowpack melted completely in place — the quantity hydrologists actually care about for water supply and flood forecasting. It depends far more on density than on depth: light, fresh powder near 100 kg/m³ gives roughly a 10:1 snow-to-water ratio, while dense, wind-packed or wet snow can approach 3:1. Two snowpacks of identical depth can therefore hold very different amounts of water.

Worked example

A 30 cm (300 mm) layer of fresh snow at 100 kg/m³ has SWE = 300 × 100 ÷ 1000 = 30 mm of water — a 10:1 ratio, or about 30 litres of meltwater per square metre. The same 30 cm of dense spring snow at 400 kg/m³ would yield 120 mm, four times as much water.

Sources: Snow water equivalent — Wikipedia

Frequently asked questions

Why is snow density more important than depth?

Water content is the product of depth and density, but density varies by a factor of five or more between fluffy powder and compacted wet snow, while a fixed depth is just one number. A shallow, dense pack can hold more water than a deep, fluffy one.

What does the "10:1 rule" mean?

It is the rough guideline that 10 cm of typical fresh snow melts down to about 1 cm of water, corresponding to a density near 100 kg/m³. It is only an average — warm, wet snow can be 5:1 or heavier, and very cold, dry snow can exceed 15:1.

How is snow water equivalent measured in the field?

Common methods include weighing a snow core of known cross-section, using a calibrated snow pillow that senses the weight of the pack, or automated sensors at monitoring stations. Each ultimately measures mass per unit area, which converts directly to SWE.

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This calculator is for educational purposes. Double-check important results before acting on them.