Required footing area is the minimum plan area of a foundation footing needed to safely transfer the structural loads (dead loads, live loads, and other applicable loads) from a column, post, or wall into the underlying soil without exceeding the soil’s allowable bearing capacity. An additional allowance of roughly 10 % is often added to the column or wall load to account for the self-weight of the footing and the soil above it. The resulting area is then used to select practical plan dimensions (square, rectangular, or circular) that satisfy both the calculated area and applicable building code minimums.
Required Footing Area (Soil Bearing Service Loads) Formulas |
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\( A_{req} \;=\; \dfrac{ P }{ q_{allow} } \) (Required Footing Area) \( A_{req} \;=\; \dfrac{ P_d + P_l }{ q_{net} } \) (Required Footing Area) |
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| Symbol | English | Metric |
| \( A_{req} \) = Required Footing Area | \(ft^2\) | \(m^2\) |
| \( P \) = Service Load on Footing | \(lbf\) | \(N\) |
| \( q_{allow} \) = Allowable Soil Bearing Pressure | \(psf\) | \(Pa\) |
| \( P_d \) = Service Dead Load | \(lbf\) | \(N\) |
| \( P_l \) = Service Live Load | \(lbf\) | \(N\) |
| \( q_{net} \) = Net Allowable Soil Pressure = \(q_{allow}\) - Soft/Footing Overburden | \(psf\) | \(Pa\) |
