Snow Water Equivalent Calculator (Snow to Rain)

This calculator converts a snow depth into the water it holds, its snow water equivalent. Enter the snow depth, the snow-to-liquid ratio and an area such as a lawn or roof; you get inches and millimetres of water and the gallons over that area once it melts.

Formula

Water equivalent (in) = snow depth ÷ snow-to-liquid ratio. Millimetres = inches × 25.4. Gallons over an area = area (ft²) × water (in) ÷ 12 × 7.4805, where 7.4805 = 1,728 ÷ 231 gallons per cubic foot.

• s: depth of fresh snow in inches
• r: inches of snow per inch of water (10 to 1 is a common rough average; wet snow is lower, dry powder higher)
• a: area in square feet

Worked Example

10 in of snow at 10 to 1 over 1,000 ft².

1. Water = 10 ÷ 10 = 1.00 in
2. In millimetres: 1 × 25.4 = 25.4 mm
3. Melt water = 1,000 × 1 ÷ 12 × 7.4805 = 623 gal

How to use the result

Use it to compare a snowfall with rain, to estimate spring melt on a lawn, or to see how much water a roof will shed. The ratio is the uncertain part: if you can, melt a measured core of snow and measure the water.

Limits and common mistakes

• The 10 to 1 ratio is a rough average; real ratios range widely with temperature and snow type.
• Old, settled snow is denser than fresh snow, so its ratio is lower.
• This is a water estimate, not a roof snow-load rating; ask a qualified professional about roof loads.

Frequently Asked Questions

How much rain is 1 inch of snow?

At 10 to 1, 0.1 in of water; at 20 to 1, 0.05 in.

Is wet snow heavier?

Yes, it holds more water per inch, so its ratio is lower (often 5 to 1 or less).

How do I measure the ratio myself?

Take a core of snow in a straight-sided can, melt it, and divide the snow depth by the water depth.

Does this give roof load?

No; it estimates water only.

in s | Snow depth | in | 10 | 0 | 500
in r | Snow-to-liquid ratio (x to 1) | ratio | 10 | 1 | 100
in a | Area | ft² | 1000 | 0
out w | Water equivalent | in | s / r | 2
out mm | Water equivalent | mm | w * 25.4 | 1
out gal | Melt water over the area | gal | a * w / 12 * 1728 / 231 | 0