How Wide Should a Footing Be?

The rule of thumb: twice the wall width

The quick answer builders use on site is that a footing should be at least twice as wide as the wall resting on it. An 8 inch foundation wall sits on a 16 inch footing, a 6 inch wall on a 12 inch footing, and so on.

The extra width on each side is called the projection, and it spreads the point load of the wall over more soil area so the ground underneath does not compress unevenly and crack the slab or wall above it. That projection has its own rule too: it must be at least 2 inches per side and cannot exceed the footing's thickness.

This 2x rule is a fast field check, not a substitute for the code table. It works for an average one-story house on average soil, but heavier construction or soft ground need more. For the exact width and the concrete volume it takes to pour it, run your numbers through the concrete footing calculator.

What the IRC actually requires by story count

Table R403.1(1) of the International Residential Code sets minimum footing widths for conventional light-frame construction based on how many stories the wall carries and the load-bearing value of the soil beneath it. More stories and weaker soil both widen the number.

These figures are prescriptive minimums for typical residential loads, not a substitute for the actual table your local jurisdiction has adopted. Always check the version your building department enforces, since some states amend the base IRC numbers.

Absolute minimums and adjustments for building width

No matter what a table, a rule of thumb, or a soil report says, IRC Section R403.1.1 sets a hard floor: a residential footing can never be narrower than 12 inches or thinner than 6 inches.

The code's width tables also assume a building 32 feet wide. If your house is narrower, the code allows the footing to shrink by 2 inches for every 4 feet under 32 feet, and to grow by 2 inches for every 4 feet over it, as long as the 12 inch floor is never crossed. A 24 foot wide cabin, for example, can trim a couple of inches off the table value.

Footing width and thickness are only half the sizing question. The depth the footing sits at below grade is a separate code rule tied to your local frost line, covered in how deep should a footing be.

How soil type changes the number

Footing width exists to keep the pressure the wall places on the soil below a safe limit, so weaker soil needs more surface area to spread that same load. When no geotechnical report is available, the IRC lets builders use a presumptive load-bearing value from Table R401.4.1 instead of testing.

Soft clay and silt are commonly assumed at around 1,500 pounds per square foot (psf), sandy clay closer to 2,000 psf, and dense sand or gravel can reach 3,000 to 4,000 psf. A wall on 1,500 psf clay needs a noticeably wider footing than the same wall on 4,000 psf gravel.

If your soil is soft, expansive, fill, or you simply are not sure, a geotechnical soil test is worth the cost. It can justify a narrower, cheaper footing on good ground, or catch a weak spot before the foundation goes in rather than after it settles.

Brick veneer, masonry, and other heavier walls

The 12/16/24 inch figures above assume standard wood-frame construction. A wall carrying more weight, such as 4 inch brick veneer over wood framing or an 8 inch solid or fully grouted masonry wall, needs a wider footing on the same soil to keep the bearing pressure within limits.

The IRC covers this with separate tables, R403.1(2) for brick veneer and R403.1(3) for masonry construction, both of which call for wider footings than the plain wood-frame table at every soil and story combination. As a rule, expect masonry and veneer walls to need roughly 25 to 50% more footing width than an equivalent wood-frame wall.

Retaining walls and tall foundation walls add lateral soil pressure on top of the vertical load, which usually calls for an engineered footing rather than a prescriptive table value.

Footings for decks, posts, and other point loads

Everything above applies to a continuous strip footing under a wall, where the load runs the length of the footing. A deck post, fence post, or column footing carries its load at a single point instead, so it is sized by diameter and tributary area rather than by doubling a wall width.

A wider, deeper strip footing and a round post footing are different calculations, even though both start from the same soil bearing logic. The specifics, including how tributary deck area drives the diameter, are in deck footing size.

For a straightforward wall footing, plug your wall width, footing depth, and length into the concrete footing calculator to get the exact concrete volume and bag count for the pour.

Frequently asked questions

References

• IRC R403.1.1 Minimum size, footings (ICC Digital Codes)
• Inspecting Footings, IRC R403 (InterNACHI)
• Shallow foundation (Wikipedia)
• ACI 332: Residential Concrete Code

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