Snow water equivalent, roof snow load and slope angle calculator (snow tube, layers, map contours, phone inclinometer, GPS)

What it does

Three small field calculators for snow, on one page, all working without internet once the page is open:

• Snow water equivalent (SWE): how much water the snowpack holds, in millimetres (1 mm is 1 kg on each square metre). Work it out from depth and density, from snow tube weighings (tube with snow, empty tube, core length) or from layers of a snow pit (thickness and density of each). The page also gives the bulk density and the ground snow load.
• Snow load on a roof: the ground load (from the SWE or a value you type) times your own roof factor, in kN/m², kg/m² and lb/ft², and the total on a roof area. It is not a design code: building codes set their own loads and factors, and roofs need an engineer.
• Slope angle: from map contours (interval, number of intervals, map distance and scale), from your phone’s tilt sensor laid on the slope, from a typed clinometer reading, or from two GPS points with elevations. The angle is shown on a 0 to 90° bar with the general band where most slab avalanches start (about 30 to 45°).

This is not an avalanche or safety forecast. Angle is only one part of avalanche terrain; the snowpack, the weather, the terrain above and the runout below all matter. Read your local avalanche forecast and take an avalanche course.

How to use it

1. SWE: pick a method. For a snow tube, type the inner diameter of the cutter and one line per sample: the tube’s weight with snow (g), empty (g) and the length of the snow core (cm). For a pit, type one layer per line, thickness then density. You can also open a CSV file of samples or layers.
2. Roof: keep “the SWE above”, or switch to typing a ground load. Type a roof factor (roof load ÷ ground load) and, if you like, the roof’s plan area.
3. Slope: pick map, phone or GPS. On a phone, press Start the tilt sensor (iPhones ask for permission), lay the phone screen up on the snow or on a ski pole laid down the fall line, and press Hold this reading. For GPS, type both points or use Use my location at each one; heights from a map or altimeter are better than phone GPS heights.
4. Download the results as CSV, or a PNG, PDF or Word report.

Worked example (made-up numbers)

• Depth and density: 85 cm of snow at 250 kg/m³ gives SWE = 212.5 mm, a ground load of 2.084 kN/m².
• Snow tube: a 4.6 cm tube (16.62 cm²) with three samples, 480, 505 and 460 g of snow over 78, 82 and 74 cm, gives SWE 288.8, 303.9 and 276.8 mm, so SWE = 289.8 mm (standard deviation 13.6 mm) and a bulk density of 372 kg/m³.
• Layers: 12 cm at 90, 25 cm at 180, 30 cm at 260 and 15 cm at 320 kg/m³ make 82 cm of snow with SWE 181.8 mm and a bulk density of 222 kg/m³.
• Roof: with the first example’s 2.084 kN/m² and a factor of 0.8, the roof load is 1.67 kN/m² (170 kg/m²); on 120 m² that is 200 kN, about 20.4 tonnes of snow.
• Map: five 10 m contour intervals across 3 mm on a 1:25 000 map: 50 m of rise over 75 m, 33.7°, in the 30 to 45° band.
• GPS: two made-up points 146.8 m apart horizontally and 95 m apart in height give 32.9°.

Tips

• Weigh the tube on the same scale with and without snow, and measure the core length, not the hole depth, if some snow fell out.
• Density above 917 kg/m³ is impossible (that is solid ice), so the page flags it: check grams, centimetres and the diameter.
• Contours hide small steep steps: a short convex roll can be much steeper than the map shows.
• A phone measures the surface right under it; measure in a few places and on the steepest part.

FAQ

Is a slope under 30° safe? No. Avalanches can start on lower slopes in some conditions, and gentle slopes are often in the path or runout of steeper ones. The band is a general fact about angles, not advice.

Why does my phone show no reading on a computer? Most computers have no tilt sensor. Type an angle from a clinometer instead.

Does anything leave my device? No. The tilt sensor, your location and your numbers stay in this page.

Last updated 2026-11-09.