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Divide the total service or factored load on the footing by the allowable net bearing capacity of the soil to get minimum contact area.
Required Area (sq ft) = Load (lbs) ÷ Bearing Capacity (psf)For a square spread footing, take the square root of required area for equal length and width. Rectangular footings divide area by the fixed dimension.
Side (ft) = √(Required Area)Verify the proposed footing dimensions produce bearing pressure at or below the allowable soil capacity. If pressure exceeds capacity, the footing is undersized.
Actual Pressure (psf) = Load ÷ (Width × Length)Updated: July 2026
A deck column carrying 4,000 lb on clay at 1,500 psf needs 2.67 sq ft minimum — a 20″×20″ pad is marginal; 24″×24″ (4 sq ft) is safer.
A 20,000 lb load on firm soil at 2,000 psf requires 10 sq ft — a 3×3.3 ft rectangular footing or 3.16 ft square. Confirm with local code and frost depth.
Enter 2 ft width with 15,000 lb load to verify actual pressure of 3,750 psf against 2,000 psf capacity — result flags undersized footing for redesign.
Preliminary sizing for spread footings supporting columns or concentrated loads on soil. Enter the factored load and allowable soil bearing capacity to find minimum footing area and check whether your proposed width and length keep bearing pressure within limits.