The calculator estimates the gross ultimate bearing capacity:
- c = soil cohesion (kPa), φ = friction angle (deg).
- q = surcharge at base = γ·Df (kPa).
- B = footing width (or diameter for circular), γ is unit weight (kN/m³).
- s, d, i = shape, depth, and inclination factors.
- Allowable (gross) bearing is qa = qult / FS.
Water table effects are handled by adjusting γ for surcharge and the Nγ term.
- Pick your footing shape (rectangular, square, strip, or circular).
- Enter soil parameters: cohesion c, friction angle φ, and unit weight γ.
- Provide footing dimensions B and L, plus embedment depth Df.
- Set the water table depth Dw to reflect site groundwater.
- Add factor of safety and applied service load P.
- Press Calculate to view results below the header.
- Download a CSV or PDF report for submittals and records.
| Case | Soil | c (kPa) | φ (deg) | γ (kN/m³) | B×L (m) | Df (m) | FS | Typical qa (kPa) |
|---|---|---|---|---|---|---|---|---|
| 1 | Dense sand | 0 | 35 | 19 | 1.5×2.0 | 1.0 | 3.0 | 250–450 |
| 2 | Medium sand | 0 | 30 | 18 | 1.2×2.0 | 1.0 | 3.0 | 150–300 |
| 3 | Stiff clay (undrained) | 60 | 0 | 17 | 1.5×1.5 | 1.2 | 3.0 | 120–220 |
| 4 | Soft clay (undrained) | 25 | 0 | 16 | 2.0×2.0 | 1.0 | 3.0 | 50–110 |
- This tool checks bearing capacity only; settlement may govern design.
- Use consistent soil parameters (effective stress for drained checks).
- Water table correction uses a practical interpolation within B below base.
- Inclination factor is a simplified reduction; use detailed methods when needed.
- For layered soils, weak strata, or eccentric loading, consult a geotechnical engineer.
1) Why bearing capacity matters
Bearing capacity is the pressure a footing can apply without shear failure. On many building sites, allowable values commonly sit around 75–400 kPa, depending on density, moisture, and confinement. This calculator screens footing options by comparing applied pressure (P/A) with an allowable bearing estimate. For preliminary sizing, many teams aim for utilization below 0.85 to absorb construction variation.
2) Inputs that drive the result
The core soil inputs are cohesion c, friction angle φ, and unit weight γ. Sands often use φ=28–38° and gravels 35–45°. Bulk unit weight typically ranges 16–22 kN/m³. For short‑term undrained clay checks, set φ≈0 and use a tested cohesion value.
3) Foundation shape and dimensions
Width B affects the third bearing term and the adjustment factors. Square and circular plans can mobilize more confined failure zones than long strips. Practical shape factors help compare rectangular, square, strip, and circular footing options consistently.
4) Embedment depth and surcharge
Embedment depth Df increases surcharge q=γ·Df and may improve capacity through depth factors. Shallow footings are often founded around 0.8–1.5 m, but use project levels when grading, excavation, or backfill changes the effective ground surface.
5) Groundwater correction with real numbers
Groundwater reduces effective unit weight. This tool uses a submerged unit weight γ′=γ−9.81 (kN/m³) when water is at or above the base, and interpolates when water lies within B below the base. Example: with γ=18, the submerged value is γ′≈8.19, which can reduce allowable bearing.
6) Factor of safety selection
Allowable bearing is computed as qa=qult/FS. Many sites use FS=2.5–3.5, adjusted for investigation quality and consequence of failure. Follow the governing design basis in the geotechnical report and applicable codes.
7) Where the soil parameters come from
Parameters are typically derived from SPT/CPT data, lab strength tests, unit weight measurements, and groundwater observations. Record the depth range used because bearing capacity is depth‑dependent. For layered or variable soils, evaluate multiple cases and document your selections. When uncertainty is high, use conservative inputs and verify during excavation.
8) Using the output responsibly
Bearing capacity is one check; settlement often governs in compressible soils. If utilization is high, consider increasing footing area, improving soil, or reducing load, then re‑run the check. Confirm final design with project load combinations, construction tolerances, and professional recommendations. Save the CSV or PDF output for reviews.
1) Should I use service load or factored load?
For allowable stress checks, use service loads so the factor of safety applies correctly. For LRFD workflows, follow your project method and do not mix formats.
2) What if my soil has φ = 0 degrees?
Set φ to 0 and enter undrained cohesion c from testing. The calculator applies appropriate bearing factors for low friction materials such as saturated clays.
3) How do I set the water table depth?
Enter the depth from ground surface to the observed groundwater level. If water is near the footing base, expect lower allowable bearing because effective unit weight decreases.
4) Why does increasing B sometimes raise capacity?
The γ·B·Nγ term grows with width, and wider foundations can mobilize larger failure zones. However, settlement may increase with width, so check both.
5) Does this tool check settlement?
No. It estimates bearing capacity only. Settlement evaluation depends on soil compressibility, stress distribution, and time effects, and should be checked using appropriate geotechnical methods.
6) What factor of safety should I choose?
Common values are 2.5–3.5, but use the value required by your geotechnical report, code, and risk profile. Higher variability typically warrants higher safety.
7) What if the result shows CHECK?
Increase footing area, reduce load, improve soil, raise embedment, or reassess soil parameters with better testing. Confirm final decisions with the geotechnical engineer of record.