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Concrete Footing Calculator

Pier and column pads sized the way R403.1.1 sizes them

Halve the soil bearing under a pier and the pad does not double. It grows by about 41 percent on a side, because the code sizes it by area and area is a square. Everybody expects the wrong answer here, in both directions.

Pier and column footing

sq ft
psf
lb
in
in
ft

Sized the way IRC R403.1.1 sizes a pier or column footing: tributary load divided by the allowable soil pressure from Table R401.4.1. Deck footings are a different table and are on the deck footing calculator.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

This figure is drawn from the pad computed above.
Your pad, and the pad on half the bearing

Worked example

A post carrying 144 sq ft of floor at 50 psf, on an 8 in pier, on 1,500 psf clay.

  1. The load is the area times the load on it. 144 sq ft at 50 psf is 7,200 lb. Tributary area is half the span to the next support in each direction, which for a post in the middle of a 12 by 12 bay is the whole 144.
  2. The area is the load divided by the bearing. 7,200 / 1,500 = 4.80 sq ft, which is a 26.3 in square pad.
  3. Now double the soil bearing to 3,000 psf. The area halves to 2.40 sq ft, and the pad goes to 18.6 in. Not 13.2. The side moved by a factor of 0.71, which is one over the square root of two.
  4. The projection is what sets the thickness. A 26.3 in pad under an 8 in pier projects 9.15 in each side, and R403.1.1 says the projection shall not exceed the thickness. So that pad needs to be at least 10 in thick, not the 6 in minimum.
  5. Push the load to 24,000 lb and the rule bites hard. That needs 16 sq ft, a 48 in pad, projecting 20 in. On an 8 in pad the projection is two and a half times the thickness, and the pad is cantilevering further than it can carry.

The number to take away is not the area, it is the thickness the projection forces. A pad sized on bearing alone and poured 6 in thick because that is the code minimum is the commonest way this calculation goes wrong.

The formula

One division and one square root, and then a limit:

A = P / q    side = √A    T ≥ (side − column) / 2
P
the load reaching this footing, including the pad and the soil on it
q
the allowable soil bearing pressure from Table R401.4.1, not a guess
√A
why the side moves with the square root and not with the load
T
the pad thickness, which R403.1.1 requires to be at least the projection

The square root is the whole of the intuition problem. Four times the load is four times the area and only twice the side, so a pad that looks barely bigger can be carrying a great deal more. It also runs the other way: a pad that looks generously oversized on plan may be only slightly oversized in capacity.

Why this is a different page from the foundation calculator

R403.1.1 contains two rules, not one. The first sends continuous wall footings to Tables R403.1(1) through (3), which is what the foundation calculator reads. The second is a single sentence and it covers this page: 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.

No table, just the arithmetic. Which is unusual for the IRC and it is why this calculation is honest: there is nothing to look up, so there is nothing to get wrong except the two inputs, and both of those are things you have to find out rather than assume.

Deck footings are neither of those. AWC DCA 6 Table 4 sizes deck footings directly from beam span and joist span, at an assumed 1,500 psf, and the deck footing calculator reads that table. Using this page for a deck will give you a defensible answer by a different route, but the prescriptive table is what an inspector will be holding.

The load is the part this page cannot check. Tributary area is geometry and you can measure it. Load per square foot is a design decision made from IRC Table R301.5 and your own dead loads, and a page that filled it in for you would be guessing at the most consequential input on it.

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.

This site publishes no price of its own. Why, and what that means for these figures.

Tributary area, and the half-span rule

Every footing carries the floor that drains to it, and the boundary between one support and the next is halfway between them. That is the whole of tributary area.

A post in the middle of a bay carries half the span in each direction, on all four sides. Between posts 12 ft apart running one way and 12 ft the other, that is 6 ft in each of four directions: a 12 by 12 square, 144 sq ft. The post is at the centre of the area it carries, not at the corner of it.

A post at the end of a run carries half as much. There is nothing beyond it, so its area stops at the beam end rather than continuing half a span further. End posts are routinely oversized because people count the same square footage for every post in the line.

And a post under a girder carries what the girder brings. If the girder is picking up joists from both sides, its tributary width is half the joist span each way, and the post takes half the girder span each way of that. Two halves multiplied, which is a quarter of the four bays around it.

The load calculator works the same idea down through a whole structure, if you want to see where the number came from before it reached the ground.

What the pad weighs, and why that is not a rounding error

The load you enter is supposed to include the footing itself and the soil sitting on top of it. On a small pad that is negligible. On a large one it is not.

Concrete runs about 150 lb per cubic foot. A 48 in square pad 20 in thick is 26.7 cubic feet, so roughly 4,000 lb before anything stands on it. Against a 24,000 lb column load that is another 17%, and it is bearing on the same soil.

The soil on top counts too, where the pad is buried. A pad four feet down with a foot of projection all round has a column of earth standing on that projection, and earth is 100 to 120 lb per cubic foot.

Neither is in the number this page starts from, because neither is known until the pad is sized, and sizing the pad is what needs the number. The honest way round it is to size once, add the pad's own weight, and size again. Two passes converge quickly.

The practical version: on anything over about a 36 in pad, add the concrete weight and re-run it.

What this page does not decide

Reinforcement. A pad wide enough for its projection to matter is bending, and bending in concrete is a job for steel. The IRC's prescriptive footings are plain concrete within the geometry limits above; go outside them and you are into ACI 318 and a designer. Nothing on this page sizes a bar.

Depth. Frost line, from Table R301.2(1), which is filled in locally and runs from nothing to several feet. It is a phone call, not a calculation.

Whether the soil is what you think. Table R401.4.1 gives presumptive values by soil class, and presumptive means until somebody looks. Below 1,500 psf the code stops presuming and requires an investigation.

Uplift and lateral load. A pad sized for downward bearing has no answer for a post in tension or a column being pushed sideways. Both are real in high wind and in seismic design categories D0 and above, and both change the footing rather than adjust it.

Frequently asked questions

How do I size a concrete footing for a post?
Divide the load the post carries by the allowable soil bearing pressure. That gives the area in square feet, and the side of a square pad is the square root of it. A post carrying 7,200 lb on 1,500 psf soil needs 4.8 sq ft, which is a 26.3 in square pad. Then check the thickness against the projection.
Does doubling the load double the footing?
No, and this is the one people get wrong. It doubles the AREA, so it multiplies the side by the square root of two, about 1.41. A pad for twice the load is 41% wider, not twice as wide. The same applies going the other way when the soil is better.
How thick does a pier footing need to be?
At least as thick as it projects past the column, per R403.1.1: the projection shall not exceed the thickness of the footing. A 26 in pad under an 8 in pier projects 9 in, so it needs to be 9 in thick or more, not the 6 in that is the code minimum elsewhere. Sizing the area and then pouring the minimum thickness is the commonest error on this calculation.
What is tributary area for a post?
The floor area that drains to it: half the span to the next support in every direction. A post in the middle of a bay with 12 ft spans both ways carries a 12 by 12 square, 144 sq ft. A post at the end of a run carries about half that, because there is nothing on the far side.
Is a deck footing sized the same way?
It can be, but the prescriptive route is different. AWC DCA 6 Table 4 sizes deck footings directly from the beam and joist spans at an assumed 1,500 psf soil, and that is the table an inspector will be holding. The deck footing calculator on this site reads it.
Should I include the weight of the footing itself?
Yes, and most calculators do not. Concrete is about 150 lb per cubic foot, so a 48 in pad 20 in thick is roughly 4,000 lb standing on the same soil as the load above it. On small pads it does not matter. Over about a 36 in pad it does.
Can I use a round footing instead of a square one?
Yes, and the area is what matters, so a round pad of the same area does the same job. The diameter for a given area is about 1.13 times the side of the equivalent square. Round is what you get from an auger, square is what you get from a form, and the choice is usually made by the tool rather than the code.

Check these numbers yourself

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.

Related calculators

This sizes the bearing area and checks the projection. It does not design the footing. A pad whose projection approaches its thickness is bending, and reinforcement, punching shear and eccentric loading are all outside the IRC's prescriptive rules and outside this page. The load per square foot is your figure and it is the input that matters most. Depth below grade is set by a local frost table. Soil below 1,500 psf requires an investigation rather than a calculation.