Footing Size
Sizes the plan area of a spread footing under a single column with a concentric load, so the soil pressure stays within the allowable bearing pressure. It gives the required area, the side of a square footing, the diameter of a round one, and the ACI minimum depth. Take the bearing pressure from a geotechnical report, or, where there is none, pick a soil class to use the presumptive values of IBC Table 1806.2 [3]. Engineers and builders use it for a first footing size before the structural design of the concrete.
The equations
ACI CODE-318-25 13.3.1.1 sizes the base area with service (unfactored) loads and the permissible soil pressure [2]:
A = P/q
B = √A
D = √(4A/π)
P is the column service load, q the allowable bearing pressure, A the required area, B the side of a square footing and D the diameter of a round one. With a gross allowable pressure, P must include the weight of the footing and the soil over it; with a net pressure, leave them out [1].
The depth of the footing above the bottom bars must be at least 6 in. (ACI 13.3.1.2). Below the bars, concrete cast against earth needs 3 in. of cover (Table 20.5.1.3.1), so a footing is seldom thinner than about 10 in. overall.
Limits and common mistakes
- Use service loads here, not factored ones. Factored loads come in later, for the footing’s shear and flexure design.
- The page assumes the load is centered. A column moment or an eccentric load makes the pressure uneven; the peak, not the average, must stay below q.
- The presumptive values do not apply to mud, organic silt or clay, peat or unprepared fill. A geotechnical report may justify a higher value.
- Bearing is only one check. Settlement, frost depth, and the punching and one-way shear in the concrete often set the size or depth.
- Round the side up to a practical size, such as the next 50 mm or 3 in.
Worked example
An interior column carries a service load of 620 kN, including an allowance for the footing and backfill. The site is silty sand and there is no geotechnical report. Size a square footing.
- Enter Column Service Load (P) 620 kN.
- Choose Soil Class Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC): 2,000 psf (95.8 kPa). The page fills in the bearing pressure.
- Press Solve. The Solution shows:
Required Bearing Area: 6.47448 m²
Square Footing Side: 2.5445 m
Round Footing Diameter: 2.87116 m
Minimum effective depth d of the bottom bars, 6 in. (ACI 318-25 13.3.1.2): 152.4 mm
Check: 2,000 psf is 95.76 kPa, and 620 / 95.76 = 6.474 m², whose square root is 2.544 m. Use a 2.6 m square footing; its average pressure is 620 / 2.6² = 91.7 kPa, under the allowable.
