Load per footing and the footing diameter your soil needs, using IRC presumptive soil bearing values
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A deck footing has one job: spread the load from its post over enough soil that the soil does not sink. The load is the deck area the post carries (half the joist span times the post spacing, plus any cantilever) multiplied by the design load. The IRC deck provisions use 40 psf live load plus 10 psf dead load. Divide that by what your soil can bear and you have the footing area.
This tool works from the presumptive bearing values in IRC Table R401.4.1, so it matches what an inspector will use when no soil report exists. Footings still have to reach below your local frost depth, and a hot tub or a roof over the deck changes the loads entirely.
Need the beam and post sizes too? Use the deck beam span calculator. To price the concrete for every hole, run the Sonotube concrete calculator, and the deck calculator handles boards, joists and fasteners.
| Soil | Bearing (psf) |
|---|---|
| Clay, sandy clay, silty clay, clayey silt (CL, ML, MH, CH) | 1,500 |
| Sand, silty sand, clayey sand, silty gravel (SW, SP, SM, SC, GM, GC) | 2,000 |
| Sandy gravel and gravel (GW, GP) | 3,000 |
| Sedimentary and foliated rock | 4,000 |
| Crystalline bedrock | 12,000 |