How Wide Should a Footing Be?
Footing width cannot be selected from wall width alone. Under the prescriptive 2021 International Residential Code, the applicable table depends on wall construction, number of stories and presumptive soil bearing value; engineered designs may use a different calculation. The calculator on this page does not design a footing. After the width and thickness are taken from the adopted code, approved plans or an engineer, it calculates concrete volume and bag count.
Key takeaways
- The 2021 IRC uses separate prescriptive tables for light-frame walls, brick veneer and masonry construction.
- R403.1.1 generally requires spread footings at least 6 inches thick and not less than 12 inches wide unless another provision applies.
- Wall construction, supported stories, building width and presumptive soil bearing value all affect the table lookup.
- For applicable tables based on a 32 foot building width, each 4 foot change adjusts tabulated footing width by 2 inches; a 24 foot building is an 8 foot change, so the adjustment is 4 inches.
- Deck and post footings are sized by diameter and tributary load, not by wall width, so they follow a different chart entirely.
Why twice the wall width is only a preliminary check
Twice the wall width is a common field rule of thumb, but it is not the general IRC sizing method and it cannot establish compliance by itself. The prescriptive code tables use wall construction, supported stories, building width and soil bearing value.
The projection is the part of the footing that extends beyond the supported wall. Under IRC R403.1.1, footing projection must be at least 2 inches and must not exceed the footing thickness. Those limits still do not determine the required bearing width.
Select width and thickness from the code adopted by the local jurisdiction, approved plans or an engineered design. Then enter those dimensions in the concrete footing calculator to calculate cubic yards and bags. The calculator does not select the width.
What the IRC actually requires by story count
Table R403.1(1) of the 2021 IRC covers conventional light-frame construction. Table R403.1(2) covers light-frame construction with brick veneer, and Table R403.1(3) covers masonry construction. Each table crosses supported stories with presumptive soil bearing value.
| Input to confirm | Why it matters |
|---|---|
| Wall construction | Selects the applicable IRC table |
| Supported stories | Changes the gravity load carried by the footing |
| Soil bearing value | Controls the area needed to spread that load |
| Building width | Adjusts the tabulated value where the table note applies |
A generic 12, 16 or 24 inch list omits these inputs and must not be used as a design table. Check the adopted code edition and local amendments before selecting a value.
Absolute minimums and adjustments for building width
Under the prescriptive rule in 2021 IRC R403.1.1, spread footings must be at least 6 inches thick. Footing width must follow the applicable table and generally must not be less than 12 inches unless another code provision applies. An engineered foundation can follow an approved design outside these prescriptive tables.
The table notes use a 32 foot building-width basis. Where the note applies, increase or decrease the tabulated footing width by 2 inches for each 4 foot change in building width. A 24 foot building is 8 feet narrower than the baseline, so the adjustment is 4 inches, subject to the table note and minimum width.
Footing width and thickness are only part of the design. Embedment and frost protection are separate requirements, 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.
State the approved footing dimensions, calculated quantities and construction scope in each quote.
Brick veneer, masonry, and other heavier walls
Brick veneer and masonry must not be estimated from a generic light-frame footing width. The IRC provides separate tables because construction weight and load path differ.
Use R403.1(2) for the light-frame-with-brick-veneer conditions within its scope and R403.1(3) for masonry construction within its scope. Compare the exact row for the supported stories and soil value instead of applying an unsupported percentage increase.
Retaining walls and tall foundation walls also resist lateral loads. Their footing design depends on wall geometry, soil, drainage, surcharge and the code provisions that apply to the system.
Footings for decks, posts, and other point loads
Everything above applies to a continuous strip footing under a wall. A deck post or column footing carries a concentrated load and follows a different sizing method based on tributary load and soil bearing.
A strip footing and a round post footing are different calculations. The inputs behind deck footing selection are explained in deck footing size, with local code or engineering still controlling the final dimensions.
After the dimensions are approved, enter wall-footing length, width and thickness in the concrete footing calculator to get concrete volume and bag count.
Frequently asked questions
What is the minimum width of a footing?
Will a 4 inch thick concrete footer hold up a house?
What is the rule of thumb for concrete footings?
How wide should a footing be for an 8 inch wall?
Sources and references
These references and structured source records support the method and material data used on this page.
- IRC R403.1.1 Minimum size, footings (ICC Digital Codes)
- Inspecting Footings, IRC R403 (InterNACHI)
- Shallow foundation (Wikipedia)
- IRC 2021 Chapter 4: Foundations and prescriptive footing tables