How Wide Should a Footing Be?

The rule floor (IRC R403.1.1)

Under the International Residential Code, a concrete footing must be at least 12 inches wide and 6 inches thick. The footing must project at least 2 inches beyond each face of the wall it supports, and that projection may not exceed the thickness of the footing. So a 6-inch-thick footing can project between 2 and 6 inches per side — which is exactly why an 8-inch wall sits comfortably on a 12-inch footing.

Minimum footing width by soil and stories

Required width grows with the building load and shrinks as the soil gets stronger. This is the widely reproduced IRC soil-bearing width table (minimum width in inches), the version most building departments hand out:

Values from IRC Table R403.1 “Minimum Width of Concrete or Masonry Footings.” The 2015–2021 IRC reorganized this table into a snow-load format (Table R403.1(1)), but the R403.1.1 minimums and projection rule above are unchanged. Always confirm against your adopted local code.

What your soil is worth

The width table keys off the soil’s presumptive load-bearing value. Where no soil test is done, the IRC lets you assume these values by soil type (IRC Table R401.4.1); a geotechnical report can justify higher:

Where soil bearing is not tested, jurisdictions commonly default to 1,500 psf. Weaker or uncontrolled fill requires a soils investigation.

Width in the field

In practice, an 8-inch foundation wall on adequate soil takes the minimum 12-inch footing; a 10-inch wall is commonly set on a 16–20-inch footing, about twice the wall width. Both keep the projection inside the code window. The heavy widths in the table — 24 to 42 inches — show up under three-story or full-masonry walls on weak soil, or wherever an engineer sizes the footing for a specific lot.

See the footing size chart for common residential dimensions side by side, get the concrete volume for any width on the concrete footing calculator, and check depth on how deep footings need to be.