Foundations

Footing Size Calculator

Size square, rectangular, or continuous strip footings from service load and allowable soil bearing pressure.

Engineering schematic

Service-load footing sizing

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

Plan and section views of a spread footing showing service load P, plan dimensions B and L, required area, and uniform allowable soil pressure. — Symbols
P or w
isolated-footing service load or strip-footing line load
B, L
required footing width and length
Areq
required soil-contact area
qallow
allowable service-level soil pressure

Result

Required footing: 8.29 ft × 8.29 ft

Design service load

275 kip

Required area

68.75 ft²

Required width

8.29 ft

Required length

8.29 ft

Step-by-step calculation

Design service load
$P_d=250\left(1+\frac{10}{100}\right)=275\ \mathrm{kip}$
Required bearing area
$A_{\mathrm{req}}=\frac{P_d}{q_{\mathrm{allow}}}=\frac{275}{4}=68.75\ \mathrm{ft²}$
Plan dimensions
$B=\sqrt{\frac{A_{\mathrm{req}}}{r}}=8.29\ \mathrm{ft},\qquad L=rB=8.29\ \mathrm{ft}$

What this means

Round both dimensions up to practical values, then verify the resulting pressure, settlement, eccentricity, punching shear, one-way shear, flexure, and detailing.

Important limitations

Confirm whether the geotechnical allowable pressure is net or gross before adding footing, backfill, or overburden loads.

Assumptions

Concentric vertical service load
Uniform contact pressure
Level foundation base
Input allowable pressure applies to the selected footing dimensions and embedment

Formula Used

A_{\mathrm{req}}=\frac{P_{\mathrm{service}}}{q_{\mathrm{allow}}},\qquad B=\sqrt{\frac{A_{\mathrm{req}}}{r}},\qquad L=rB

Step-by-Step Example

A square footing carrying 250 kip with a 10% additional-load allowance and 4 ksf allowable pressure requires 68.75 ft², or approximately 8.29 ft square before practical rounding.

Engineering Notes

Use service loads with an allowable bearing pressure and factored loads with a factored geotechnical resistance; do not mix design formats.
Confirm whether the geotechnical value is net or gross and which footing width, embedment, groundwater condition, and settlement criterion it assumes.
Round dimensions up, then recalculate actual service pressure using the selected footing size.
Complete eccentricity, settlement, sliding, uplift, punching shear, one-way shear, flexure, development, and durability checks as applicable.

FAQ

Should I use service or factored loads?

Use service loads with allowable bearing pressure. A strength or resistance-factor design requires compatible factored loads and geotechnical resistance.

Does the calculated area include settlement?

Only indirectly if the entered allowable pressure was established from settlement criteria. Verify total and differential settlement separately.

How should I round the footing size?

Round dimensions up to practical construction increments, then check the actual pressure and all structural limit states using the final geometry.

Related Engineering Guides

Spread Footing Sizing ExampleFollow service-load sizing, dimension rounding, and the final pressure check.Spread Footings: Applications and LimitationsReview the checks that follow preliminary area sizing.Foundation Design WorkflowCoordinate reactions, pressure, movement, structural design, and construction requirements.