Free snow load calculator
Find out how many pounds — and how many psf — of snow are pressing on your roof before a thaw or the next storm, updated live, as you type.
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ASCE 7-22 snow density estimates. For structural safety decisions, consult a licensed engineer. Do not use this to make roof-clearing decisions alone.
Results are estimates. Consult a professional.
What is roof snow load?
Roof snow load is the weight that accumulated snow puts on a roof or any flat surface, measured as total pounds and as pounds per square foot (psf). This snow load calculator turns three numbers you can read off the roof — snow depth, the roof area, and how wet or packed the snow is — into the weight that snow is pressing down with, so you can compare it against what the roof was built to carry.
It matters because snow is far heavier than it looks. A foot of light powder is one thing; the same foot after it settles, soaks up rain, or partly melts and refreezes can weigh several times more. When that weight climbs past the load the roof was designed for, the result is sagging, leaks at the seams, or — in the worst case — collapse. Knowing the number before a thaw or the next storm is the whole point of running the figures.
How the snow load calculator works
Snow weight is just volume times density. The calculator takes the snow depth in inches, the area in square feet, and a density in pounds per cubic foot, then divides by twelve to convert the depth from inches into feet so the units line up. The same math, expressed per square foot, gives the psf figure that building codes use.
The inputs explained
Three numbers go in. Depth and area are easy to read directly; density is the one worth thinking about, because it is what separates harmless powder from a dangerous load.
Snow depth
How deep the snow sits on the roof, in inches. Measure on the flat part of the roof, away from drifts — wind can pile snow two or three times deeper against walls, parapets, and roof valleys, and those drift zones carry the heaviest local loads.
Roof or area
The footprint the snow is sitting on, in square feet. For a sloped roof, use the horizontal (plan) area the roof covers, not the longer slope length — codes apply snow load to the projected horizontal area.
Snow density
How much each cubic foot of that snow weighs. The calculator offers three presets — fresh/fluffy at 5 lb/ft³, settled/packed at 20 lb/ft³, and wet/icy at 30 lb/ft³ — because density is what swings the answer most. The same depth of wet snow weighs six times more than fresh powder.
Snow density: why a foot of snow can weigh six different things
Fresh snow is mostly air, so it is light. As it sits, the crystals settle and pack down; rain, melt, and refreezing drive water into the pack and push the density up sharply. A foot of fresh powder and a foot of old, rain-soaked snow look similar from the ground but can differ in weight by a factor of six or more.
| Snow type | Density (lb/ft³) | What it looks like |
|---|---|---|
| Fresh / fluffy | ~5 (3–5) | Just-fallen dry powder; light enough to sweep. The calculator's "fresh" preset. |
| Settled / packed | 10–20 | Snow that has sat for days and compacted under its own weight. The "settled" preset uses 20. |
| Wet / windpacked | 20–30 | Heavy, dense snow — wind-packed, partly melted, or rain-soaked. The "wet / icy" preset uses 30. |
| Solid ice | ~57 | Ice from melt-and-refreeze cycles. Heavier than any snow preset; beyond the calculator's range. |
Density figures follow USDA Natural Resources Conservation Service (NRCS) snow-survey guidance. The calculator computes with its three presets (5, 20, 30 lb/ft³); the ~57 lb/ft³ ice row is shown as the heavy extreme a refrozen pack can approach, not a value the tool calculates with.
Typical snow densities — fresh snow near 5 lb/ft³, settled snow in the 10–20 range, wet or wind-packed snow toward 20–30, and solid ice near 57 lb/ft³ — follow the snow-water-equivalent relationships documented by the USDA Natural Resources Conservation Service snow-survey program.psf versus total pounds: which to use
The calculator returns two figures, and they answer different questions. The total pounds tells you the entire weight the structure is holding — useful for picturing the load or comparing two roofs. Pounds per square foot (psf) is the one that matters for safety, because building codes and roof design loads are written in psf.
So read the total pounds to grasp the sheer scale of what is up there, and read the psf to judge whether the roof is in trouble. The next section puts that psf number against the loads typical roofs are built for.
When snow load becomes dangerous
Every roof is designed for a specific snow load set by its local building code, and that ground-snow figure varies enormously by region — from near zero in the warm South to well over 100 psf in mountain and lake-effect zones. Most ordinary residential roofs in snow country are designed to carry somewhere around 20 to 30 psf, but the only authoritative number for a given building is the design load on its plans or in its local code.
| Calculated psf | Reading | What to do |
|---|---|---|
| Below 20 psf | Light for most roofs | Typically well within a code-built roof's capacity; keep an eye on drifts and on wetting from rain or melt. |
| 20–30 psf | Approaching common design loads | Near the range many residential roofs are designed for. Watch the roof and clear it if more snow or rain is coming. |
| Above 30 psf | Heavy — verify the design load | May meet or exceed an ordinary roof's capacity. Check the structure's rated load and consider safe removal or a professional assessment. |
These bands are general orientation, not a code. Actual roof capacity depends on its design, age, condition, and local snow code; drifted areas and flat roofs that pond meltwater carry far higher local loads. Source: FEMA Snow Load Safety Guide (FEMA P-957), implementing ASCE 7-22 Chapter 7.
Warning signs that a roof is overloaded include new sagging, doors or windows that suddenly stick, fresh cracks in interior drywall or masonry, and creaking or popping sounds. If you see any of these, treat the building as unsafe and get a structural engineer involved. When snow melts and the water runs off, the related rainfall-to-gallons calculator helps estimate the volume that meltwater becomes for drainage and harvesting.
A worked example using the snow load calculator
A late-winter storm drops snow that then takes on rain, leaving an 18-inch wet, packed layer across a 1,200 ft² roof. The homeowner wants to know whether that load is something to worry about before the next system arrives.
Step 1 — Enter the three figures
Depth 18 inches, area 1,200 ft², and density set to "wet / icy" (30 lb/ft³) because the snow is heavy and rain-soaked.
Step 2 — Convert depth to a load per square foot
Using the psf line, psf = (18 ÷ 12) × 30 = 45 psf. That is the weight on every square foot of the roof.
Step 3 — Scale up to the whole roof
Multiply by the area: load = 45 × 1,200 = 54,000 lb — about 27 tons of snow sitting on the structure.
What this calculator does and does not tell you
This tool estimates the weight of snow on a surface from depth, area, and density. It does not certify that a roof is safe. Whether a given load is acceptable depends on the structure's design load, its age and condition, the roof's slope and shape, and where snow has drifted — none of which a weight calculation can see.
- Density is the biggest unknown. The three presets are typical values, not measurements. Real snow varies; when in doubt, choose the heavier preset — underestimating density underestimates the risk.
- Drifts carry far more than the average. Wind piles snow against walls, parapets, and in roof valleys, where local loads can be two to three times the flat-roof figure. Measure depth in the deepest drift if you are checking for danger.
- Use the projected horizontal area. For sloped roofs, enter the plan (footprint) area, not the longer slope length, to match how codes apply snow load.
- It is an estimate, not an engineering certification. If the calculated psf approaches the roof's design load, or you see sagging, sticking doors, or cracking, stop and consult a licensed structural engineer.
Used with that caveat in mind, the snow load calculator is a fast way to know roughly how much weight is overhead before deciding whether to clear it. For a related winter-weather figure, the wind chill calculator shows how cold that same snowstorm will feel to anyone working on the roof.
Frequently asked questions about the free snow load calculator
About this snow load calculator
This snow load calculator estimates the weight of snow on a roof or area — in total pounds and pounds per square foot (psf) — from its depth, footprint, and density, using the ASCE 7-22 / FEMA P-957 depth x area x density / 12 relationship. It runs entirely in your browser: nothing you enter is sent to a server or stored. Use it to gauge whether a load is approaching a roof's design capacity, but treat the result as an estimate — it weighs the snow, it does not certify that the roof is safe.