Foundations

Footing Contact Pressure Calculator

Calculate biaxial corner pressures, eccentricities, kern status, and allowable-pressure utilization for a rigid rectangular footing.

Engineering schematic

Biaxial footing contact pressure

Static symbolic figure. Enter project values in the calculator fields below.

Rectangular footing plan with width and length axes, vertical load, biaxial moments, kern diamond, four corner pressures, and a linear contact-pressure diagram. — Symbols
P
vertical service load at the contact-area centroid
MB, ML
moments about the footing width and length axes
eB, eL
resultant eccentricities in the plan directions
qmin, qmax
minimum and maximum calculated corner pressures

Result

Maximum contact pressure: 4.16 ksf

Average pressure

3.75 ksf

Maximum corner pressure

4.16 ksf

Minimum corner pressure

3.34 ksf

Width eccentricity

0.067 ft

Length eccentricity

0.1 ft

Full contact

Yes

Allowable-pressure utilization

83.2%

Step-by-step calculation

Average contact pressure
$q_0=\frac{P}{BL}=\frac{300}{8\times10}=3.75\ \mathrm{ksf}$
Load eccentricities
$e_B=\frac{M_L}{P}=0.067\ \mathrm{ft},\qquad e_L=\frac{M_B}{P}=0.1\ \mathrm{ft}$
Corner pressure equation
$q=q_0\pm\frac{6M_B}{BL^2}\pm\frac{6M_L}{LB^2}$
Pressure range
$q_{\min}=3.34\ \mathrm{ksf},\qquad q_{\max}=4.16\ \mathrm{ksf}$
Combined kern check
$\eta_k=\frac{6e_B}{B}+\frac{6e_L}{L}=0.11leq1.0$

What this means

The resultant lies within the biaxial kern for the entered moments, so the linear elastic full-contact pressure distribution remains compressive.

Assumptions

Rigid rectangular footing
Linear pressure distribution
Service-level vertical load and moments
Moments are resolved about the footing centroidal axes

Formula Used

q=\frac{P}{BL}\pm\frac{6M_B}{BL^2}\pm\frac{6M_L}{LB^2}

Step-by-Step Example

An 8 ft by 10 ft footing carrying 300 kip with 30 kip-ft and 20 kip-ft moments remains in full contact; the four corner pressures follow from the two linear moment gradients.

Engineering Notes

Resolve loads and moments at the centroid of the footing-soil contact area before using the equations.
For biaxial loading, full contact requires all calculated corner pressures to remain compressive; the kern is diamond-shaped in eccentricity space.
A negative calculated pressure identifies loss of contact. Do not use the negative value as soil tension; perform a partial-contact equilibrium analysis.
Compare service-level maximum pressure with a compatible allowable bearing pressure and evaluate sliding, overturning, uplift, settlement, and structural design separately.

FAQ

What does full contact mean?

It means the linear pressure distribution is compressive at every corner. It does not by itself establish adequate bearing, settlement, sliding, or overturning performance.

What if the minimum pressure is negative?

Soil cannot transmit tension. The footing has partial contact under the entered load combination, so the contact area and pressure distribution must be recomputed from equilibrium.

Can I enter factored loads?

Only if the pressure limit is a compatible factored geotechnical resistance. The default interpretation uses service loads and allowable pressure.

Related Engineering Guides

Biaxial Footing Pressure ExampleCalculate eccentricities, kern status, four corner pressures, and utilization.Eccentric Loads and Partial ContactUnderstand when the linear full-contact solution stops being physical.Spread Footing Design ChecksPlace the pressure calculation in the complete design workflow.