Soil Pressure Calculator

1. Enter footing plan dimensions B and L in meters.
2. Enter axial load P in kN (downward positive).
3. Add moments Mx and My if the load is eccentric.
4. Optionally include overburden using γ and Df.
5. Press Submit to see results above the form.
6. Use the CSV or PDF buttons to export your report.

Contact pressure context

Shallow foundations transfer structural actions into soil as bearing pressure. This calculator reports average pressure q̄ = P/A and elastic corner pressures. Use the outputs to screen allowable bearing, estimate contact stress variation, and flag edge concentrations that can drive settlement or tilting.

Loads, moments, and eccentricity

Axial load P is applied at the footing centroid, while Mx and My represent overturning about the centroid axes. The induced eccentricities are ex = My/P and ey = Mx/P. For example, My = 120 kN·m on P = 1200 kN gives ex = 0.10 m. As eccentricity increases, pressure shifts toward one side and unloading can occur opposite.

Linear elastic distribution

For a rectangular base, the assumed distribution is linear: q(x,y)=P/A+(Mx·y/Ix)+(My·x/Iy). The section properties are Ix=B·L³/12 and Iy=L·B³/12. Corner values combine ±B/2 and ±L/2, giving qmax and qmin for rapid checks. Because kN/m² equals kPa, the unit conversion is direct.

Kern and no‑tension behavior

The kern limits are B/6 and L/6. If both eccentricities remain inside these limits, the entire base stays in compression under the linear model. If the check fails or qmin becomes negative, uplift is possible and the soil cannot carry tension. In that situation, designers often adopt a compression-only assumption and re-check stability.

Effective area estimate

The effective dimensions Be = B − 2|ex| and Le = L − 2|ey| provide a practical compression-only approximation. The calculator reports Ae = Be·Le and q̄eff = P/Ae. If Be or Le becomes small, stress can rise sharply; treat values above typical service ranges such as 150–300 kPa as a prompt for larger footing, lower load, or improved ground.

Interpreting results for design

Compare qmax to allowable bearing capacity at the governing depth and drainage condition, then apply your code’s load combination rules. Review qmin for potential loss of contact and check overturning, sliding, and settlement with geotechnical parameters. Use the contour plot to communicate stress gradients to the team, and attach CSV or PDF exports to calculations for review and traceability. Always confirm inputs align with the footing centroid and sign conventions before finalizing the design.

1) What does qmax represent?

qmax is the highest elastic contact pressure at the footing corners, including moments and optional overburden. It helps check bearing capacity and local stress concentration.

2) Why can qmin become negative?

Large eccentricity can unload part of the base in the linear model, producing negative pressure. Soil cannot resist tension, so uplift or reduced contact area may occur.

3) When is the kern check important?

If |ex| ≤ B/6 and |ey| ≤ L/6, compression is expected across the entire base. Outside the kern, consider compression-only methods and stability checks.

4) What is the effective area output used for?

Be and Le reduce the contact dimensions to approximate compression-only behavior. q̄eff is the average pressure on that reduced area and is useful for conservative screening.

5) Should I include overburden pressure?

Include γ×Df when you want total base stress at depth, such as for comparing to in-situ vertical stress. Exclude it when checking net bearing against allowable values.

6) Are these results final for construction?

Use them for quick design checks and documentation. Final sizing should follow project load combinations, geotechnical recommendations, and settlement evaluations appropriate to the site.