What this number means — and what it doesn't
Snow load is depth times density. The density is the part people get wrong: fresh dry powder weighs very little, but the same depth of settled, rain-soaked or refrozen snow can weigh three to five times as much, and a solid ice layer is heavier still because ice is close to the weight of water. That's why roofs fail during a thaw-refreeze or a rain-on-snow event rather than during the storm itself — the depth on the roof barely changes while the weight climbs. The densities behind the options above are typical ranges, not measurements of your snow. If you want a real number, the reliable method is to cut a core of the full snow depth with a container of known area and weigh it, from a safe spot on the ground.
This calculator estimates the current weight of snow on a roof. It is not a code check. The ground snow load for your location and the roof design snow load your building was engineered to carry are code-defined values that come from your local building code and a structural design, and they include factors this page has no way to know: roof slope and how well snow slides off it, exposure and wind, thermal conditions, importance category, and drift and sliding loads that pile snow far deeper against walls, valleys, dormers and lower roof sections than the average depth suggests. A drift can easily be several times the load calculated here over the area it covers.
Safety: this is a rough estimate only, and it cannot tell you whether a roof is safe. If you are worried about a roof — sagging, cracking or popping noises, doors or windows that suddenly stick, cracks in interior finishes, or heavy drifting — leave the building and call a structural engineer, your building department, or the fire service. Do not climb onto a snow-loaded roof to measure it or clear it: falls and roof collapses during clearing cause serious injuries and deaths every winter. Snow removal from a roof is work for people with the right equipment and training, done from the ground or with proper fall protection, and uneven clearing can make loading worse. Verify all design loads against your local code and a licensed engineer.