Foundations · Bearing area
Spread footing size from load and soil pressure.
Enter the load and the allowable soil bearing pressure from your geotechnical report to see the footing size that keeps soil pressure within it.
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Spread footing size: how it works
Soil pressure is load divided by area. The footing must be big enough that the pressure it puts on the soil stays at or below the allowable bearing pressure. The calculator adds your self-weight allowance to the load, divides by the allowable pressure to get the area needed, and for a column footing takes the square root to get the side. The size is then rounded up to the step you choose and the pressure is recomputed at that size. Allowable bearing capacity is the ultimate capacity divided by a factor of safety, and on soft soil it can be set by allowable settlement instead.
Worked example, imperial. A 40 kip column, 2,000 psf allowable pressure, 10% allowance. Load with self-weight is 44 kips, so the area needed is 22.0 ft2 and the exact side is 56.3 in. Rounded up to the next inch that is 57 in, and the soil pressure at 57 in is 1,950 psf.
Worked example, metric. A 150 kN column, 100 kPa allowable pressure, 10% allowance. Load with self-weight is 165 kN, so the area needed is 1.65 m2 and the exact side is 1,285 mm. Rounded up to the next 25 mm that is 1,300 mm, and the soil pressure at 1,300 mm is 97.6 kPa.
What this does not check. Bearing area is one check among several. It does not cover footing thickness, shear, bending, reinforcement, settlement, frost depth, groundwater, eccentric loads or uplift. For the concrete volume of the footing, use the Concrete Calculator.
Frequently asked questions
How do I size a spread footing?
Divide the load, including an allowance for self-weight, by the allowable soil bearing pressure to get the area needed. For a square column footing take the square root to get the side. For a wall strip footing the area per unit of length is the width.
Where do I get the allowable bearing pressure?
From the geotechnical report for the site, or from the value your local building authority permits. The example value in the calculator is a placeholder, not a recommendation for your soil.
Is bearing area the only thing that sizes a footing?
No. Footing thickness, shear, bending, reinforcement, settlement, frost depth and groundwater all matter, and an engineer checks them. This tool covers the bearing-area check only.
What is the difference between ultimate and allowable bearing capacity?
Ultimate bearing capacity is the theoretical pressure at which the soil fails. Allowable bearing capacity is the ultimate value divided by a factor of safety, and on soft soil settlement can set the limit instead.
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Not professional advice
This estimates the bearing area only and is not a foundation design. Real footings need an engineer to confirm soil capacity, footing depth below frost, thickness, reinforcement, settlement and load combinations under your local building code. Never size a structural footing from this page alone. The default pressure is an example, not a safe value for your soil. Not professional advice.