All 648 cells of IRC Tables R403.1(1), (2) and (3)
There is not one footing table in the IRC, there are three, and they disagree by a factor of nearly two on the same soil. Every cell in all three is drawn for a 32 foot wide house, and a footnote tells you what to do about that.
Foundation footing size
ft
ft
in
ft
Every cell is read from IRC Tables R403.1(1) through R403.1(3) as printed in the 2018 Seattle Residential Code. Footnote b is applied to your house width. Frost depth is not on this page, because it is local.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Width, thickness and projection, to scale
Worked example
A 40 ft by 32 ft house, light frame, two storeys over a crawl space, on 1,500 psf clay with 8 in foundation walls.
The width of the house is the width in the table, so footnote b does nothing here. Thirty-two feet is the basis every cell is drawn for. The table gives 16 in wide by 6 in thick and that is the answer.
Now build the same house 40 ft wide instead. That is four adjustment steps of two feet, so footnote b adds 8 in of width and 4 in of thickness. The footing goes from 16 x 6 to 24 x 10, which is 2.5 times the concrete.
Now build it in masonry rather than light frame. Different table. Two storeys plus a basement on the same 1,500 psf soil reads 35 x 12 on R403.1(3) against 23 x 6 on R403.1(1).
Put those two together and the prescriptive table runs out. A 40 ft wide masonry house on that soil comes to 43 in wide and 16 in thick, which puts the projection at 17.5 in on a 16 in footing. R403.1.1 says the projection shall not exceed the thickness, so that combination is not in the tables and needs a design.
And a soil at 1,200 psf is not a smaller footing. Footnote a forbids extrapolation and the table starts at 1,500. Table R401.4.1 sends anything softer to a soils investigation.
The useful output is not the width. It is knowing which of the three tables your house is in, and whether the footnote has taken you off the end of it.
The formula
A table lookup, an interpolation, and the footnote:
the cell from whichever of the three tables matches your wall type, storeys, floor type and snow load
house width − 32
how far your house is from the basis every cell was drawn for
/ 2
footnote b works in two-foot steps, so this is the number of steps
max(6, ...)
footnote b's own floor: never less than 6 in thick
Then two limits from R403.1.1 that footnote b never mentions. The projection may not be less than 2 in, which stops the downward adjustment producing a footing narrower than the wall on it. And the projection may not exceed the thickness, which is what a wide house in masonry runs into.
Three tables, and how far apart they are
R403.1(1), R403.1(2) and R403.1(3) are three different tables for three different walls, and most footing calculators quote one of them without saying which. Light frame, light frame with brick veneer, and cast-in-place concrete or fully grouted masonry.
On the worst cell in the set they are not close. Three storeys plus a basement, 1,500 psf soil, 20 psf snow: light frame is 25 x 8 inches, brick veneer is 32 x 11, masonry is 43 x 17. The masonry footing is 72% wider and more than twice as thick as the light-frame one, on the same dirt, under the same roof.
Every cell in all three is drawn for a 32 foot wide house. Footnote b says so and tells you the adjustment: two inches of width and one inch of thickness for every two feet. On a 40 ft house that is 8 inches and 4 inches, which on a small footing is more than double the concrete. This page applies it. Almost nothing else does.
And the tables have an edge. Footnote a permits interpolation and forbids extrapolation, so 1,750 psf is a legitimate reading between two columns and 1,200 psf is not a reading at all. Table R401.4.1 footnote b is explicit about what happens below 1,500: a soils investigation, not a table.
Read from Seattle's adopted code, which prints the model text in full. codes.iccsafe.org returns HTTP 403 to any automated request, so the ICC's own site cannot be quoted from here. Your jurisdiction may have amended any of it, and the frost depth that sets how deep the trench goes is local by construction.
Prices for ready-mix and bagged concrete
The published figures for ready-mix and bagged concrete are on the concrete calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.
At the heavy end they are nowhere near each other. Three storeys plus a basement on 1,500 psf soil: light frame 25 x 8, brick veneer 32 x 11, masonry 43 x 17. The masonry footing is 72% wider and 2.1 times as thick.
At the light end they converge. A single-storey slab-on-grade house is 12 x 6 in all three, because at that load the footing is being sized by the minimums rather than by the load. Which is why a calculator that only ever gets tested on a small house looks right and is not.
The middle is where it matters, and the middle is most houses.
Every cell is drawn for a 32 foot wide house
This is the one that gets left out. It is not buried in a commentary, it is footnote b, printed under all three tables:
Based on 32-foot-wide house with load-bearing center wall that carries half of the tributary attic, and floor framing. For every 2 feet of adjustment to the width of the house, add or subtract 2 inches of footing width and 1 inch of footing thickness (but not less than 6 inches thick).
So the table is not a lookup, it is a lookup plus an adjustment, and the adjustment is linear in how far your house is from 32 feet. Four feet wider is two steps: 4 in more width, 2 in more thickness.
On a small footing that is not a trim, it is a multiplier. A 16 x 6 in footing on a 32 ft house becomes 24 x 10 in on a 40 ft house. Per foot of wall that is 96 square inches of section against 240, so two and a half times the concrete for a house 25% wider.
The reason is in the same footnote. The cells assume a load-bearing centre wall carrying half the attic and floor framing, so the exterior footings each take a quarter of the house. Widen the house and that quarter grows, on the same length of footing.
Which also means an open plan is outside the assumption. Take the centre bearing wall away and the load it was carrying has to go somewhere, and where it goes is the exterior footings the table just sized for half the job.
Two limits in R403.1.1 that the footnote never mentions
The minimum width, W, and thickness, T, for concrete footings shall be in accordance with Tables R403.1(1) through R403.1(3) and Figure R403.1(1) or R403.1.3, as applicable. The footing width shall be based on the load-bearing value of the soil in accordance with Table R401.4.1. Footing projections, P, shall be not less than 2 inches (51 mm) and shall not exceed the thickness of the footing. The size of footings supporting piers and columns shall be based on the tributary load and allowable soil pressure in accordance with Table R401.4.1.
Footnote b floors the thickness at 6 in and says nothing about the width. Follow it downward on a narrow house and it produces footings that cannot exist: a 24 ft house takes a 12 in footing to 4 in wide, which is narrower than the wall standing on it.
The projection rule is what stops that. P shall be not less than 2 inches, on each side, so the footing is never narrower than the wall plus 4 inches however far the footnote adjusts down. On an 8 in foundation wall that is a 12 in floor, and it is the reason 12 in is the smallest width printed in any of the three tables.
And the same sentence sets a ceiling. P shall not exceed the thickness of the footing, which stops a wide thin footing cantilevering further than it can carry. This is the limit a large masonry house runs into: adjust a 35 x 12 in footing up for a 40 ft house and you get 43 x 16, where the projection is 17.5 in against a 16 in thickness. That combination is not in the tables. It is a design.
The two rules are in the same section as each other and in a different place from the footnote, which is presumably why calculators apply neither.
The table has an edge, and below it there is no table
Soil bearing is the input people guess at, and it is the one the code is least willing to let you guess.
Footnote a to the footing tables: interpolation allowed, extrapolation is not allowed. So 1,750 psf is a legitimate reading between the 1,500 and 2,000 columns. 1,200 psf is not a reading at all, and a calculator that gives you a bigger footing for it has invented something the code declines to.
Where the building official determines that in-place soils with an allowable bearing capacity of less than 1,500 psf are likely to be present at the site, the allowable bearing capacity shall be determined by a soils investigation.
That is the answer for soft ground: a soils investigation. And R403.1 adds the other half, which is that whatever the number turns out to be, it has to be measured on the ground the footing will actually sit on:
Footings shall be supported on undisturbed natural soils or engineered fill.
Undisturbed natural soil or engineered fill. Backfill from last year's excavation is neither, whatever it tests at.
Three tables, four snow loads each, nine story-and-floor combinations, six soil columns. Every cell on this page is read out of that document rather than recomputed, and the module that holds them refuses to build if any cell falls outside a plausible footing or if a row ever asks for a wider footing on stronger soil.
It is one city's adopted code printing the model text. That is the best available from here, because codes.iccsafe.org returns HTTP 403 to automated requests and the ICC's own text cannot be quoted. Seattle amends parts of the IRC, and so does your jurisdiction. Where a value matters, ask which edition your building department has adopted and what they have changed.
Two things on this page are local by construction and are not here. The frost depth that sets how deep the trench goes, from Table R301.2(1), and the seismic design category, which changes the footing requirements above SDC C.
Frequently asked questions
How wide should a house footing be?
It depends which of the three IRC tables your house is in, and every calculator that gives one answer has picked a table without telling you. A two-storey light-frame house over a crawl space on 1,500 psf soil is 16 in wide by 6 in thick. The same house in fully grouted masonry with a basement is 35 x 12. And both figures assume a 32 ft wide house.
Why does the footing table depend on how wide my house is?
Because a wider house puts more floor and roof load on the same two exterior footings. Footnote b to every R403.1 table says the cells are based on a 32-foot-wide house and instructs you to add 2 in of width and 1 in of thickness for every 2 ft over that, or subtract the same going down, never below 6 in thick. A 40 ft house needs 8 in more footing width than the table prints.
What is the minimum footing thickness in the IRC?
Six inches. That figure appears twice: as the smallest thickness printed in the tables, and as an explicit floor in footnote b so the downward adjustment cannot go below it. There is no matching floor on the width in that footnote, which is why R403.1.1's projection rule has to do that job.
What is footing projection?
The part of the footing sticking out past the wall standing on it, on each side. R403.1.1 says it shall be not less than 2 inches and shall not exceed the thickness of the footing. That gives a wall on an 8 in foundation a footing between 12 in and 8 plus twice the thickness, and it is the rule that tells you when the prescriptive tables have run out.
What soil bearing value should I use?
Not a guess. Table R401.4.1 gives presumptive values by soil class, from 1,500 psf for clay up to 12,000 for crystalline bedrock, and the building official can require a test. If the soil is softer than 1,500 psf the code stops giving you a table and requires a soils investigation.
How deep does a foundation footing have to go?
Below the frost line, and this page will not tell you where that is. R403.1.4.1 sends you to Table R301.2(1), which is filled in jurisdiction by jurisdiction and runs from nothing in the south to several feet in the north. It is a phone call to the building department, not a calculation.
Does this include the footing under interior columns?
No. Those are not in these tables at all. R403.1.1 says footings supporting piers and columns are sized from the tributary load and the allowable soil pressure, which is a different calculation and is on the footing calculator on this site.
Check these numbers yourself
2018 Seattle Residential Code, Chapter 4 FoundationsSeattle Department of Construction and Inspections. Read 2026-09-09. Prints IRC Chapter 4 in full, including Tables R403.1(1) through R403.1(3), Section R403.1.1 and Table R401.4.1. Used because codes.iccsafe.org returns HTTP 403 to any automated request. All 648 cells of the three tables are read from it, along with Section R403.1.1, Table R401.4.1 and footnotes a and b.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Prescriptive tables, not a design. These cells assume a load-bearing centre wall carrying half the attic and floor framing, undisturbed natural soil or engineered fill, and a house of ordinary proportions. An open plan with no centre bearing wall, a soil below 1,500 psf, a seismic design category of D0 or higher, or a projection that exceeds the footing thickness all put the job outside them and into an engineer's hands. The depth below grade is set by a local frost table and is not on this page.