How to use the isolated footing size calculator
Sizing an isolated spread footing takes two steps. First, find the net allowable bearing pressure: start from the allowable value in the geotechnical report and subtract the weight of the footing concrete (150 pcf times thickness) and the backfill above it (unit weight times the remaining depth). Then divide the service load D + L by that net pressure to get the required plan area, and round the dimensions up to the nearest 3 in for the selected length-to-width ratio.
The tool re-checks the service bearing pressure for the rounded dimensions against the allowable value, then reports the factored net pressure from 1.2D + 1.6L divided by the provided area. That factored pressure is the input for two-way punching shear, one-way shear, and flexural reinforcement per ACI 318 Chapter 13, where the footing self-weight and backfill are excluded because the soil carries them directly.
Footings with applied moment or eccentric load, footings whose stress bulbs overlap, and sites with a high water table and uplift all need separate checks, and settlement has to be verified in addition to bearing. IBC 1806.2 presumptive values are a screening tool only, not a substitute for a soils report. Have a licensed structural engineer confirm the final footing thickness and reinforcement.
Frequently Asked Questions
Part of the pressure the soil can carry is already used by the footing concrete and the backfill sitting on top of it. Dividing the column load by the remaining net allowable pressure keeps the plan area from coming out too small.
A geotechnical report governs. Without one, IBC Table 1806.2 presumptive values are often used: 12,000 psf crystalline bedrock, 4,000 psf sedimentary rock, 3,000 psf sandy gravel, 2,000 psf sand and silty or clayey sand, and 1,500 psf clay or silt.