Estimate the ultimate and allowable bearing capacity of soil under a strip, square, circular or rectangular footing using Terzaghi’s (1943) general bearing capacity equation. Enter the footing shape and size, depth of embedment, soil unit weight, cohesion and friction angle, and a target factor of safety, and the calculator applies the correct bearing capacity factors and shape factors for your footing and works out the allowable net bearing pressure — including an optional correction if the water table sits at or above the footing base.
Switch between metric and imperial units at the top of the form; every result converts with it.
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This is a preliminary geotechnical screening tool, not a substitute for a site-specific geotechnical investigation and report. Always have a licensed geotechnical engineer confirm soil parameters and the final design.
How the calculation works
- Bearing capacity factors. Nc, Nq and Nγ are read from Terzaghi’s classic table by soil friction angle φ.
- Shape factors. Discrete factors are applied for strip, square and circular footings; rectangular footings use the standard B/L interpolation between the strip and square cases.
- Ultimate bearing capacity. qult = shapec·c·Nc + q·Nq + shapeγ·0.5·γ·B·Nγ, where q is the effective overburden pressure at the footing base.
- Water table correction. If the water table is above or within B below the footing base, the effective unit weight used in the surcharge and self-weight terms is reduced accordingly.
- Allowable pressure. qallow = qult / FS, using the factor of safety you specify (3 is the common default for foundations).
What to have ready
- Footing shape (strip, square, circular or rectangular) and trial dimensions
- Depth of footing embedment below grade
- Soil unit weight, cohesion and friction angle from a geotechnical report
- Depth to the water table, if it is within a footing width of the base
- Target factor of safety
Notes and limitations
Terzaghi’s equation is a general shallow-foundation method and does not account for depth factors, inclination or eccentric loading — for those cases use a more complete method (e.g. Meyerhof/Hansen/Vesic) or consult a geotechnical engineer. Soil parameters should come from an actual geotechnical investigation, not assumed values. This tool estimates bearing capacity only; settlement, which often governs allowable pressure for larger footings on compressible soils, is not checked here. Final acceptance of the design rests with the responsible engineer.
Soil Bearing Capacity Reference Table — Terzaghi / IBC Table 1806.2
| Soil / Rock Type | Allowable Bearing Pressure (psf) | IBC Table 1806.2 | Notes |
|---|---|---|---|
| Crystalline bedrock | 12,000 | Class 1 | Granite, gneiss, trap rock |
| Sedimentary rock | 4,000 | Class 2 | Limestone, shale, sandstone |
| Sandy gravel / gravel (GW, GP) | 3,000 | Class 3 | Dense to medium dense; GW/GP classification |
| Sand — coarse / medium dense (SW, SP) | 2,000 | Class 4 | Compact to medium dense; above water table |
| Fine sand — loose / below water table | 1,500 | Class 4 | Susceptible to liquefaction in seismic zones |
| Silt (ML) — compacted | 1,500 | Class 5 | Verify moisture sensitivity; frost-susceptible |
| Inorganic clay — stiff (CL, CH) | 2,000 | Class 5 | Stiff to hard; N > 8 blows/ft SPT |
| Inorganic clay — soft | 500–1,000 | Class 5 | Requires geotechnical investigation |
| Organic silt / clay (MH, OH) | Not suitable | Class 6 | Highly compressible; must be removed or surcharge |
| Fill (uncontrolled) | Not suitable | Class 6 | Requires geotechnical report; engineered fill only |
Source: IBC 2021 Table 1806.2 (Presumptive Load-Bearing Values) and Terzaghi (1943) bearing capacity theory. Geotechnical investigation required for all significant structures.
Soil Bearing Capacity Calculator (Terzaghi) FAQ
What is the Terzaghi bearing capacity equation?
Terzaghi’s (1943) bearing capacity equation gives the ultimate bearing capacity of a shallow foundation: q_ult = c·N_c + q·N_q + 0.5·γ·B·N_γ, where c is the soil cohesion (psf), N_c, N_q, and N_γ are dimensionless bearing capacity factors that depend on the internal friction angle φ, q = γ·D_f is the overburden pressure at foundation depth, γ is the unit weight of soil (pcf), B is the footing width, and D_f is the depth of foundation below grade. The allowable bearing capacity is then q_allow = q_ult / FS, where FS is the factor of safety, typically 2.5–3.0 for foundations. For strip footings, the general shape factors are 1.0; for square or circular footings, Terzaghi’s modified equations apply.
What is the difference between ultimate and allowable bearing capacity?
Ultimate bearing capacity (q_ult) is the maximum load per unit area a soil can support before shear failure occurs. Allowable bearing capacity (q_allow or q_a) is the safe working pressure — the design value used to size footings — calculated by dividing q_ult by a factor of safety (FS) of typically 2.5 to 3.0: q_allow = q_ult / FS. Settlement must also be checked; sometimes the allowable pressure is governed by settlement limits (typically 1 inch total or 0.75 inch differential for most structures), which can produce a lower allowable value than the shear-failure check alone. Geotechnical engineers report both and recommend the controlling value.
What is the IBC presumptive soil bearing value, and can I use it without a geotechnical report?
IBC Table 1806.2 provides presumptive allowable bearing pressures for different soil and rock classifications — for example, 3,000 psf for sandy gravel (GW/GP) and 2,000 psf for sand (SW/SP). Under IBC Section 1806.2, these presumptive values may be used without a site-specific geotechnical investigation only when the building official determines the soil classification is appropriate and the structure meets certain size and risk thresholds. For any structure over 2–3 stories, in high seismic zones, or on potentially problematic soils (clay, fill, organic materials), a full geotechnical investigation is required — and in most commercial projects, the design engineer will require one regardless.
How does the water table depth affect bearing capacity?
The water table significantly affects the effective stress in soil and therefore its bearing capacity. When the water table is at or above the base of the footing (D_w ≤ D_f), the effective overburden pressure q in Terzaghi’s equation is reduced by the buoyant unit weight (γ’ = γ_sat − γ_w ≈ 62.4 pcf). When the water table is within a depth B below the footing base, the soil unit weight γ in the last term of Terzaghi’s equation is reduced proportionally. For saturated clays, pore water pressure buildup during rapid loading governs undrained bearing capacity (φ = 0 method, using c = s_u undrained shear strength). A geotechnical report should specify the seasonal high water table depth.
What is the typical factor of safety for foundation design?
A factor of safety (FS) of 2.5 to 3.0 is standard for shallow foundation bearing capacity design under normal loading conditions. FS = 3.0 is used where site investigation data is limited or soil variability is high; FS = 2.5 is used where thorough site investigation has been performed and soil parameters are well established. For temporary loading conditions or under wind or seismic combinations, some codes allow the allowable bearing pressure to be increased by one-third (FS effectively reduced to ~1.87–2.25), acknowledging the reduced probability of extreme load coinciding with shear failure. Deep foundations (piles, drilled piers) typically use separate FS criteria from 2.0 to 2.5 per ASCE 7 and AASHTO.