Describe what you see — soil color, texture, moisture, slope — and the calculator identifies the most likely IRC R401.4.1 soil class + the presumptive load-bearing PSF + when to require a soil test from a geotechnical engineer. 8 soil types from crystalline bedrock (12,000 psf) down to organic soil (must remove), 4 moisture-condition reductions, slope adjustment, and a visual identification helper. The output is the final PSF value to use on the Footing Diameter Calculator. Most US sites default to 2,000 psf (sand or gravelly clay — the IRC presumption that needs no test).
Mixed soil — sand + gravel + some clay binding. The IRC default presumptive value (2,000 psf). Most US urban + suburban sites fall here.
Sonotube + Bigfoot bell-base sizing depends on soil PSF. Enter 2,000 psf there to size your footings.
IRC R401.4.1 presumptive load-bearing values. Site-specific soil testing recommended above ~200 sqft decks or when adding hot tubs / outdoor kitchens. Geotechnical engineer test runs $400-800 — worth it for projects with concentrated loads.
How to use
8 IRC R401.4.1 classes: crystalline bedrock (granite — 12,000 psf), sedimentary rock (limestone — 4,000 psf), sandy gravel well-graded (3,000 psf), sand or gravelly clay (2,000 psf — the IRC default), clay or clay silt (1,500 psf), soft clay or fill (UNSUITABLE), uncontrolled fill (UNSUITABLE), organic soil (must remove).
If you're not sure what soil type matches your site, pick a visual clue: rocky and can't dig with shovel = sedimentary or bedrock; yellow/brown with gravel = sandy gravel; tan/brown with stones = sand or gravelly clay (default); gray sticky clay = clay or clay silt; dark with roots = organic (remove); soft mushy near water table = soft clay or fill (unsuitable).
Dry / moist = no reduction (the ASCE/IRC default). Wet (poor drainage, pooling) reduces capacity 15%. Saturated (water table at footing depth) reduces 40% — strongly recommends soil test + drainage improvement.
Slope under 5% = no reduction. 5-15% = 10% reduction (slight lateral concern). 15-30% = 25% reduction (geotech evaluation recommended). 30%+ = 40% reduction + soil test required.
Toggle 'has soil test' on and enter the tested PSF from your geotechnical engineer report. Tested values typically range 1,500-5,000 psf and override the presumptive default.
Bottom of results shows the final PSF after all adjustments. Feed this number to the Footing Diameter Calculator to size sonotube + Bigfoot bell-base. Most US sites default to ~2,000 psf — the IRC presumption that doesn't require any test.
Material guide
Three honest paths. Composite wins the 25-year math for most homeowners, wood wins on upfront cost, and PVC is unbeatable around water. Each card below answers in one glance — recalculate the bill of materials by clicking a brand in the picker above.
How we calculate
The Soil Bearing Calculator answers the question every deck builder has at the dig stage: 'what soil bearing PSF should I use for the footing math?' Most deck calcs just ask you to enter a PSF value and assume you know. This one works the other direction — describe what you see (soil color, texture, moisture, slope), and it identifies the most likely IRC R401.4.1 soil class + presumptive PSF + when to require a soil test from a geotechnical engineer. 8 soil types (crystalline bedrock 12,000 psf → organic soil 0 psf), 4 moisture conditions with capacity reductions, slope adjustment, and a visual identification helper that suggests the soil class from your observations. Feeds the Footing Diameter Calculator + Deck Load Calculator with the PSF value to use.
Presumptive bearing values per IRC 2021 R401.4.1 Table. Visual identification guide adapted from AASHTO M-145 + USCS soil classification. Moisture reduction multipliers from ASCE 32-01 + general geotechnical practice. Slope adjustment based on simplified slope-stability factors — full slope-stability analysis requires a geotechnical engineer.
Presumptive PSF from IRC R401.4.1 Table. Moisture mult: dry/moist 1.0, wet 0.85, saturated 0.6. Slope mult: <5% 1.0, 5-15% 0.9, 15-30% 0.75, 30%+ 0.6. Multiply all factors.
If you have a soil test from a geotechnical engineer, use that value directly — it includes site-specific adjustments. Otherwise the calculator returns the adjusted presumptive value.
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