A fast, preliminary read on how wide a continuous or spread footing tends to run for a given load and soil bearing capacity. Final footing design has to be sized and stamped by a licensed engineer.
Every result is a planning estimate, not a quote or an engineering determination. Real pricing depends on soil conditions, access, existing structure, engineering requirements, and your jurisdiction’s current fee schedule. For structural questions we coordinate with a licensed engineer — we do not perform engineering ourselves. Get a site-specific estimate before committing to a budget.
Two numbers drive the width: how much load lands on the footing, and how much pressure the soil underneath can take. A continuous footing carries the wall above it measured in pounds per linear foot, so its required width is simply load divided by bearing capacity. An isolated pad carries a post or column as a single point load, so it needs an area, and the side of a square pad is the square root of that area.
The soil number is the one people guess at, and it is the one that moves the answer most. Bay Area ground is not uniform. Parts of the East Bay flats and the Santa Clara Valley sit on expansive clay that swells and shrinks with moisture. Shoreline areas from Alameda to Foster City include bay mud and engineered fill. Hillside lots in Los Gatos, Oakland, and the Peninsula bring slope setbacks and daylighting requirements that can force a deeper or stepped footing regardless of what a width calculation says. A geotechnical report replaces all of that guessing with a design value, and on most permitted work in this region you will need one anyway.
The California Residential Code publishes prescriptive minimum footing sizes for conventional light-frame construction, commonly cited as 12 inches wide by 6 inches thick for one story, 15 by 6 for two, and 18 by 8 for three. Those are floors, not targets. When the load calculation asks for less than the minimum, the minimum governs and you build to it. When the calculation asks for more, the load governs and the footing grows. Either way the engineer confirms which one applies and adds reinforcing, dowels, and embedment depth that no width calculator produces.
Mistakes we see most often on plans that come to us for pricing:
Once a stamped detail exists, we handle the layout, excavation, forming, rebar, inspection, and pour for new home foundations, additions, and ADUs across the Bay Area.
No. This is a planning aid that shows how load and soil bearing capacity interact. A permitted footing needs a size, thickness, reinforcing schedule, and embedment depth from a licensed engineer, based on your soil report and your actual loads.
Use the value in your geotechnical report. If you do not have one yet, 1,500 to 2,000 psf is a conservative placeholder for planning in much of the Bay Area, but expansive clay, bay mud, and fill are common enough here that a real report frequently changes the design.
We do not perform engineering. We are a licensed foundation and concrete contractor. We coordinate with a licensed engineer, price the work from stamped plans, and build to those plans through inspection.
That is normal for lightly loaded one and two story walls on decent soil. The prescriptive minimum is a floor set by code, so when the load-based width lands below it, the minimum controls and the footing is built to the larger dimension.
Tell us what you're building — a new home, an ADU, an addition — and we'll get back to you with next steps, usually within one business day.
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