Deck Footing Size Calculator

How this works, and what it is reading

Almost every concrete calculator asks you for a size and gives you a volume. This one runs the other way, because with a deck footing the size is the thing you do not know. The load on a post comes from how much deck it holds up, the soil can only take so much pressure, and the footing has to be wide enough to bridge the difference. The 2018 edition of the International Residential Code put that arithmetic into a table so nobody has to do it, and the 2021 edition refined it. That table, R507.3.1, is what this calculator reads.

There is one thing you have to supply that no calculator can know: your local frost depth. Table R301.2 is printed blank in the model code, for each jurisdiction to fill in when it adopts. Seattle writes in 12 inches. Massachusetts writes in 48. There is no national number, so the field above is empty on purpose and your building department has the answer in one phone call.

Tributary area is the whole ball game

Tributary area is the patch of deck a single post carries. The code defines it in one line and then leaves you to it. Footnote (e) to the table reads: “Area, in square feet, of deck surface supported by post and footings.” Section R507.3 adds that footings shall be sized to carry the imposed loads “as shown in Figure R507.3”, and Figure R507.3 is a drawing. The IRC publishes no formula and no worked example.

The American Wood Council's DCA 6 does give equations, in its Appendix B, and its Table 4 and Table B3 have to agree with each other. Working back from the cells they share gives this, which is also just the geometry of the supported rectangle:

Inner post: tributary area = beam span × (half the joist span + the joist overhang)
Corner post: tributary area = (half the beam span + the beam overhang) × (half the joist span + the joist overhang)

The half-joist-span term is why a ledger does not earn you a smaller footing. On an attached deck the joist is a simple span from the ledger to the beam, so half its load goes into the house wall and half goes to the beam. On a freestanding deck a second beam replaces the ledger and each beam still takes half. The per-footing number does not change. What changes is that a freestanding deck has two lines of footings instead of one.

Two measuring conventions from DCA 6 are worth getting right, or you will be off by a beam half-width. The joist length is “not the design span of the joist, but is from the ledger face to either the center point of the beam, if there is an overhang, or to the outside face of the rimboard if there is not an overhang.” The beam span is measured “from either centerline of post to centerline of post, if there are overhangs, or to the outside edges of the deck, if there are no overhangs.”

Note also that DCA 6's own Table 4 applies the inner-post value to every post, which makes it conservative at the corners. If you tell this calculator a post is a corner post it will give you a smaller footing than DCA 6 Table 4 does. That is correct, and it is a difference you should be able to explain to an inspector.

The load, and the snow trap

A deck is designed for 40 psf of live load under Table R301.5, and the deck tables assume 10 psf of dead load on top of it. Table R507.4's footnote (b) states it plainly: “10 psf dead load. Snow load not assumed to be concurrent with live load.” So the ordinary design load is 50 psf.

The trap is snow. Section R507.1 says a deck is designed for “the live load required in Section R301.5 or the ground snow load indicated in Table R301.2, whichever is greater.” Whichever is greater, not both. A 50 psf ground snow load does not make the design load 90 psf. It makes it 60: ten pounds of dead load plus the fifty. People add the two and end up pouring a footing half again as big as the code asks for.

The table has four load blocks, and they are the 40 psf live case plus ground snow loads of 50, 60 and 70 psf. Above 70 psf there is no row, and footnote (a) prohibits extrapolating one, so the calculator refuses rather than guessing.

One more thing that does not belong in the load. Guards are not 40 psf. Table R301.5 gives guards and handrails a 200-pound concentrated load and guard in-fill components 50 pounds, applied to the component. Stairs are 40 psf plus a 300-pound concentrated load on a tread. None of those figures feed footing sizing; they size the guard post and its connection. Do not add them here.

IRC Table R507.3.1, the 40 psf live load block

Each cell gives three numbers: the side of a square footing, the diameter of a round footing, and the thickness, all in inches. The calculator above reads from exactly this data, so the table and the tool cannot disagree.

Read the 40 square foot row at 2,000 psf and you have the single most useful fact on this page. An ordinary deck bay on ordinary soil wants a 14 inch round footing. The largest form tube most people can buy off a shelf is 12 inches. So the tube alone does not make it, and that is not a judgement call or a rule of thumb, it is the code table. It is also why the calculator asks what tube you plan to use and tells you what it carries.

The snow blocks move the answer faster than people expect. That same 40 square foot bay on 1,500 psf clay wants 16 inches round at 40 psf live, 17 inches at 50 psf ground snow, 19 at 60 and 20 at 70. At the far corner of the table, 160 square feet on clay under a 70 psf snow load, it is a 40 inch round footing 15 inches thick, which is 10.9 cubic feet or 0.4 cubic yards of concrete per post. The calculator reads all four blocks.

These values are from the 2021 IRC as reproduced in state and municipal adoptions, which are public law: the 2021 Seattle Residential Code, the Massachusetts Residential Code, 10th edition, and WAC 51-51-0507. Check them against the printed edition your jurisdiction has adopted before you pour: some states amend the table. Minnesota Rules 1309.0507 is the clearest example, applying a 12 by 12 by 6 inch floor to every deck footing and keeping a 2,500 psf soil column that the 2021 model code dropped.

Interpolating between rows is allowed. Footnote (a) says so in six words: “Interpolation permitted, extrapolation not permitted.” The calculator interpolates on tributary area and tells you when it has, and it refuses to go past the ends. Fine Homebuilding's write-up of the table works a soil interpolation the same way: at 2,250 psf and 100 square feet, read between the 2,000 psf round footing and the 2,500 psf one and use 20 inches.

You do not need a soil test, but you do need to pick honestly

This is the part people assume is harder than it is. Section R401.4.1 says it in one sentence: “In lieu of a complete geotechnical evaluation, the load-bearing values in Table R401.4.1 shall be assumed.” You may assume a bearing value with no test at all.

What you assume, though, is the value for the soil class, not a number you like the look of. Table R401.4.1 reads: crystalline bedrock 12,000 psf; sedimentary and foliated rock 4,000; sandy gravel or gravel 3,000; sand, silty sand, clayey sand, silty gravel or clayey gravel 2,000; clay, sandy clay, silty clay, clayey silt, silt or sandy silt-clay 1,500.

1,500 psf is the default for a reason that goes beyond caution. It is the bottom row of the table and it is also the code's own trigger: “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.” Below 1,500 the IRC stops tabulating and starts requiring an investigation, which is why the calculator refuses a lower value rather than sizing something off the end of the chart.

Claiming 2,000 or 3,000 psf is a claim about what your soil is, and an inspector may ask you to back it. If you are digging and the spoil is gray sticky clay, you are on the 1,500 row whatever you would prefer.

How deep, and the freestanding exception

Two separate rules, and the 2021 restructuring made the difference matter.

R507.3.2, minimum depth, applies to every deck footing: “Deck footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface.” Undisturbed is the operative word. Twelve inches into fill you placed last year is not twelve inches.

R507.3.3, frost protection, is the one with the conditional opening clause: “Where decks are attached to a frost-protected structure, deck footings shall be protected from frost by one or more of the following methods: 1. Extending below the frost line specified in Table R301.2. 2. Erecting on solid rock. 3. Other approved methods of frost protection.”

Read the first six words again. As the 2021 IRC is written, frost protection is triggered by attachment to a frost-protected structure. A genuinely freestanding deck is not caught by R507.3.3 and owes only the twelve inches. The 2018 edition got to the same place by a different route, stating the frost requirement flatly and then excepting free-standing decks.

Freestanding in the code's sense means not attached to the house at all: no ledger, and no lateral hold-down devices tying the deck to the building. If there is a ledger, R507.3.3 applies and you are digging to the frost line. Plenty of jurisdictions also delete the exception outright, so confirm it locally rather than arguing it on site.

And there is a case where you need no footings whatsoever. R507.3 carries two exceptions: free-standing decks whose joists sit directly on grade over their entire length, and free-standing decks where the joists bear directly on precast pier blocks at grade, the deck is no more than 200 square feet, and the walking surface is no more than 20 inches above grade within 36 inches of the edge.

Does a deck footing need rebar? No

This surprises people, so read it carefully: R507.3 and Table R507.3.1 say nothing about reinforcement. A plain concrete deck footing sized off the table is compliant on its own. The only footing reinforcement the IRC mandates is in R403.1.3, and that section applies in Seismic Design Categories D0, D1 and D2. Outside those categories there is no code requirement for a bar, a cage or a mat in a deck footing, and some states delete R403.1.3 entirely in adoption.

Footnote (f) is the reason the thickness column exists in the form it does: “Minimum thickness shall only apply to plain concrete footings.” The tabulated thickness is what plain concrete needs to spread the load without cracking. A reinforced footing may legitimately be thinner, if somebody designs it.

If you want steel in there anyway, that is your call and it is cheap, but the geometry is unforgiving in a small footing. Concrete cast against the ground needs three inches of cover, so most of a 12 inch pier is cover. Our rebar calculator covers spacing, lap lengths and what a grid actually costs, and the slab reinforcement guide gets into where the code does and does not ask for steel.

Square or round, and the projection rule

The table publishes both, so take whichever is easier to build: a square footing is a plywood box, a round one is a tube or a bell form, and above about 20 inches that means a purpose-made form rather than anything off a shelf. Which takes less concrete flips as the footing grows, because the code's round diameter is generous against the equal-area square at small sizes and tight at large ones. At 40 square feet and 2,000 psf the 14 inch round is the bigger pour; at 160 square feet and 1,500 psf the 31 inch round is the smaller one. Toggle the shape and the calculator will tell you.

The rule that catches people is the projection limit in R403.1.1: “Footing projections, P, shall be not less than 2 inches and shall not exceed the thickness of the footing.” Both halves bite. A footing must stick out at least two inches past the post or pier it carries, and it may not stick out further than it is thick. That is the reason the thickness column climbs with the footing size rather than sitting at six inches forever. A 36 inch pad under a 12 inch pier projects 12 inches on each side, so it has to be 12 inches thick, not six. Footnote (d) adds a separate rule for masonry: “If the support is a brick or CMU pier, the footing shall have a minimum 2-inch projection on all sides.”

Footnote (c) sets the floor at the small end: “Footing dimensions shall allow complete bearing of the post.” That is why the 5 square foot row bottoms out at a 7 inch square rather than something smaller. A 6x6 post is actually five and a half inches, so seven inches is a bearing dimension, not a soil calculation.

The post, and where the IRC and DCA 6 fall out

Table R507.4 caps deck post height, and in the 2021 edition it does it by load block, species group, post size and tributary area. The absolute ceiling is 14 feet in every case, and the practical ceiling is often far lower. A southern pine 4x4 at 40 square feet of tributary area is good to 13 feet 8 inches. The same 4x4 at 160 square feet is good to 6 feet 2 inches. A redwood or western cedar 4x4 at 120 square feet and above is NP, not permitted, at any height.

The calculator checks this for the 40 psf live load block, which is the block it can state cell for cell. For the snow blocks it says so and sends you to the printed table rather than inventing values, because the snow blocks push many more cells to NP and a plausible-looking guess would be worse than no answer.

Then there is a genuine disagreement between the two documents builders use. DCA 6 does not permit a 4x4 deck post at all. Its limitation 3 reads: “Minimum post size is 6x6 nominal and maximum post height shall be in accordance with Table 4.” Its own commentary explains why: “IRC section R407.3 specifies a minimum 4x4 (nominal) wood column size; however, it would often be overstressed in applications covered in this document. Requiring a minimum 6x6 post in DCA 6 provides adequate bearing for beams.” The IRC permits the 4x4 within its table; the wood industry's own guide says do not. If your inspector works to DCA 6, the argument is already settled.

On the connection, R507.4.1 accepts either hardware or embedment: “Where posts bear on concrete footings in accordance with Section R403 and Figure R507.3, lateral restraint shall be provided by manufactured connectors or a minimum post embedment of 12 inches in surrounding soils or concrete piers.” With an exception you should read if your site is at all suspect: “Where expansive, compressible, shifting or other questionable soils are present, surrounding soils shall not be relied on for lateral support.”

Why DCA 6's footings come out bigger

Cross-check this calculator against DCA 6 and you will find DCA 6 about an inch larger for the same tributary area. There is a clean reason: DCA 6 includes the footing's own weight and the IRC table does not. At 100 square feet and 1,500 psf the IRC gives a 22 inch square and DCA 6's Table B3 gives 23. Work it out: a 23 by 23 by 11 inch footing is 3.37 cubic feet, which at 150 pounds per cubic foot is 505 pounds. Add that to the 5,000 pound deck load, divide by 1,500 psf, and you need 3.67 square feet, which is a 23 inch square. The extra inch is the concrete carrying itself.

Neither is wrong, and the IRC's is the one written into law. Two other differences matter more in practice. DCA 6's limitation 9 says “This document does not apply to decks which will experience snow loads, snow drift loads, or sliding snow loads that exceed 40 psf”, so it is void exactly where the IRC table keeps working up to 70 psf. And its limitation 1 scopes it to decks “attached to the house to resist lateral forces”, so it does not cover a freestanding deck at all, while R507.3.1 covers both.

Where the table stops, and what the calculator refuses

A tool that always returns a number is not being helpful when the code has run out. These are the boundaries, and the calculator says which one you hit rather than extrapolating past it.

The hot tub case is worth a word because the arithmetic is not close: a filled tub is hundreds of pounds per square foot on a structure designed for fifty, and no footing size fixes that. Our hot tub slab guide covers the slab route.

Local amendments are the other thing to watch. Washington State and Seattle raise the deck live load to 60 psf, which is why that option is in the calculator. Minnesota sets a 12 by 12 by 6 inch minimum for every deck footing. Neither is in the model code.

Once you have the size

The footing size is the hard part; the rest is ordinary volume work. If you are forming the pier with a tube, the Sonotube and form tube calculator takes the tube diameter and the length and adds the pad or bell at the base, which is where a surprising share of the concrete goes. For a plain dug hole with no form, the post hole calculator handles the annulus around the post, and fence post depth covers the non-structural case where no code table applies and frost is the governing number. For a continuous footing rather than pads, use the footing calculator, which also carries the IRC footing width table and a tapered shape for a flared base.

On the concrete: the footings themselves are small, so bagged mix almost always wins. Six 14 inch footings 6 inches thick come to 3.2 cubic feet, which is six 80 lb bags. It is the piers on top that move the number: put those six footings under 12 inch tubes running 3 feet down to a frost line and the pour is 17.3 cubic feet, or 0.64 cubic yards, which starts to look like a short load. The bag calculator works the crossover and how much a truck holds covers the short load fee that makes small deliveries expensive. For strength, DCA 6 assumes 2,500 psi for its footing tables while the bagged products and the code reports for footing forms specify 3,000: the concrete PSI guide covers the difference.