Spread Footing Size Calculator (Isolated Footing Area by Bearing Pressure)
Size a square or rectangular isolated column footing from the service load, any moment and the allowable soil bearing pressure — net bearing, required area, plan dimensions and maximum and minimum soil pressure.
How it works
Net allowable pressure qnet = qa − γ·Df · required area A = P / qnet · square side B = √A (L = ratio × B), rounded up to 5 cm or 3 in · with a moment, e = M/P and qmax,min = P/(B·L) × (1 ± 6e/L) while e ≤ L/6; beyond the kern qmax = 2P / [3B(L/2 − e)] · the size grows until qmax ≤ qnet.
Footings are sized with service (unfactored) loads against the allowable bearing pressure from the geotechnical report. Because the footing and the backfill above it also press on the soil, their weight (γ·Df) is subtracted from the gross allowable pressure to get the net pressure available for the column load. A moment shifts the resultant off-centre: while it stays inside the middle third (the kern, e ≤ L/6) the pressure varies linearly; outside it, part of the footing lifts off, which most codes avoid. This sizes the plan area only — thickness, punching and one-way shear, and reinforcement are then designed with factored loads (ACI 318 Chapter 13).
Worked example
An 800 kN column load on soil with qa = 200 kPa, a 1.5 m deep base and 20 kN/m³ of soil and concrete: qnet = 200 − 20 × 1.5 = 170 kPa, so the required area is 800/170 = 4.71 m² and √4.71 = 2.17 m, rounded up to a 2.20 m × 2.20 m footing (4.84 m²). The soil pressure is 800/4.84 = 165.3 kPa, 97.2% of the net allowable.
Sources: ACI 318-19 Chapter 13 — Foundations (13.3.1.1: footing area from service loads and permissible soil pressure) · Shallow foundation — bearing pressure and eccentric loading
Frequently asked questions
Why subtract the weight of soil and concrete?
The allowable bearing pressure is a gross value at the base. The footing slab and the backfill on top of it already use part of it, so only qa − γ·Df is left for the column load. Using the gross value would make the footing too small.
Should I use factored or service loads to size a footing?
Service loads. The allowable bearing pressure already contains a factor of safety (usually 3 on ultimate bearing capacity). Factored loads are used afterwards for the structural design — thickness, shear and reinforcement.
What happens when the eccentricity exceeds L/6?
The resultant leaves the middle third, the pressure at one edge becomes zero and part of the footing lifts off the soil. The calculator enlarges the footing until the resultant is back inside the kern, which is the usual design requirement.
What allowable bearing pressure should I use?
The value from a geotechnical investigation. Without one, building codes give presumptive values — IBC Table 1806.2 allows 1,500 psf (72 kPa) for clay and 2,000–3,000 psf (96–144 kPa) for sand and gravel — which are conservative.
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This calculator is for educational purposes. Double-check important results before acting on them.