Snow Load Calculator: Roof and Deck Snow Weight
A snow load calculator finds the total weight of snow on a roof, deck, or other surface using its depth, area, and snow density, in pounds or kilograms.
Snow Load Calculator
The total weight of snow on the surface
Visualization
Calculation Formula
Snow Load = Depth × Area × Density
- Depth: 6 in
- Area: 10 × 10 = 100.00 ft²
- Density: 12.5 lb/ft³ (Medium Snow)
Documentation
A snow load calculator estimates how much a layer of snow weighs on a roof, deck, or other flat surface. It works out the total weight from three numbers: the depth of the snow, the size of the surface, and the snow's density.
What snow load means
"Snow load" is the weight that accumulated snow places on a structure. It matters because roofs and decks are built to carry only so much weight before they sag or fail. Fresh snow is light and fluffy. Old, wet, or wind-packed snow is much heavier for the same depth. A calculator that only asks for depth, without accounting for density, can badly underestimate the true weight.
This calculator reports the total weight of snow sitting on the surface, in pounds or kilograms. It does not report pressure per square foot on its own, though that value can be found by a simple division (see below).
How to calculate snow load
The calculator uses this formula:
1Total weight = Depth × Length × Width × Density
2Before multiplying, it converts the depth into the same unit as the length and width, so the units cancel correctly:
- Imperial units: depth is entered in inches and converted to feet by dividing by 12. Length and width are in feet, density is in pounds per cubic foot (lb/ft³), and the result is in pounds.
- Metric units: depth is entered in centimeters and converted to meters by dividing by 100. Length and width are in meters, density is in kilograms per cubic meter (kg/m³), and the result is in kilograms.
The area of the surface is simply length multiplied by width.
Snow density values
Snow density depends on how fresh, wet, or compacted the snow is. The calculator offers three presets, plus a custom option:
| Snow type | Density (imperial) | Density (metric) | Typical condition |
|---|---|---|---|
| Light snow | 4.4 lb/ft³ | 70 kg/m³ | Fresh, dry powder |
| Medium snow | 12.5 lb/ft³ | 200 kg/m³ | Settled snow, a few days old |
| Heavy snow | 25 lb/ft³ | 400 kg/m³ | Wet, rain-soaked, or wind-packed snow |
| Custom | user-entered | user-entered | Any measured or estimated value |
Heavy wet snow is roughly six times denser than light powder, so the same depth of heavy snow weighs about six times as much.
Worked example
Take a flat roof section 10 feet long and 10 feet wide, covered by 6 inches of medium (settled) snow.
- Convert depth to feet: 6 ÷ 12 = 0.5 ft
- Find the area: 10 × 10 = 100 ft²
- Multiply by density: 0.5 × 100 × 12.5 lb/ft³ = 625 lb
In metric units, take a surface 4 meters long and 3 meters wide, covered by 15 centimeters of medium snow.
- Convert depth to meters: 15 ÷ 100 = 0.15 m
- Find the area: 4 × 3 = 12 m²
- Multiply by density: 0.15 × 12 × 200 kg/m³ = 360 kg
Converting the result to pounds per square foot
Building codes and roof capacity ratings are usually written as a pressure, such as pounds per square foot (psf) or kilopascals, not as a total weight. To convert the calculator's result into that form, divide the total weight by the surface area.
Using the first example above: 625 lb ÷ 100 ft² = 6.25 psf. That figure, not the 625 lb total, is the one to compare against a roof's rated load capacity.
Frequently asked questions
Does the calculator give a pressure value like psf, or a total weight?
It gives a total weight, in pounds or kilograms. Divide that number by the surface area (length × width) to get a pressure value such as pounds per square foot.
Why does snow density matter so much?
Because the same depth of snow can weigh very differently depending on how wet or compacted it is. A foot of fresh powder is light. A foot of wet, rain-soaked snow can weigh several times as much. Using the wrong density preset can give a badly wrong result.
How can snow depth be measured accurately?
Measure straight down to the surface with a ruler or stick at several spots, away from drifts piled up by wind, and average the readings. Drifted snow can be much deeper in one corner than across the rest of the surface.
Is a custom density value ever needed?
Yes. The three presets are general estimates. A more exact density, if it is known from a local weather report or a direct measurement, gives a more accurate total.
Is this calculator a substitute for a structural engineer?
No. It only multiplies depth, area, and density. It does not account for a roof's design capacity, its slope, drifting patterns, or the condition of its framing. A structural engineer or the local building code is the right source for whether a specific roof is safe under a given snow load.
What is the difference between ground snow load and roof snow load?
Ground snow load is the depth and weight of snow sitting on open, level ground. Roof snow load is usually lower, because wind and roof slope carry some snow away and heat from the building can melt some of it. Engineers apply adjustment factors to convert a ground snow load figure into a design value for a roof.