Deck Footing Size Calculator
Footing diameter, side and thickness read straight off AWC DCA 6-2015 Table 4, keyed on beam span and joist span, then scaled for your actual soil bearing capacity and for corner posts. Round or square, with frost depth and the concrete volume for the whole deck returned alongside.
Built for the moment a deck permit set comes back asking what the footings are sized for, when the honest answer is that nobody checked the soil.
Same deck, three soils, three footings
- 12 ft beam span, 14 ft joist span, round footing, interior post
- 1,500 psf, the table assumption: 30 in diameter
- 2,000 psf, a firmer site: 26 in
- 1,000 psf, soft clay or fill: 37 in
Eleven inches of difference on the same deck. Soil bearing capacity is a property of the site, not of the drawing, so this tool asks for it instead of assuming it.
Four things that decide the footing
Tributary area deck footing sizing, done by the table
An interior post carries half the beam span each way times half the joist span, and DCA 6 Table 4 indexes on beam span and joist span for exactly that reason, so you match your spans to the table rather than computing an area in square feet. The design basis behind those numbers is 40 psf live plus 10 psf dead. Beam spans run 6 to 18 ft and joist spans bucket at 10, 14 and 18 ft, and anything past those limits leaves the table for an engineered design.
Soil bearing capacity 1500 psf, and why it is an input
Table 4 note 2 states the tabulated sizes assume 1,500 psf soil bearing and 150 pcf concrete. That figure is the IRC presumptive minimum for unknown soil, not a measurement of your site. If your building department presumes higher, or a soils report gives you a number, the footing gets smaller; on fill or soft clay it gets larger or the AHJ sends you to a geotech. Never assume above 1,500 psf without something in writing.
Why the dimension moves by a square root
Bearing capacity is a pressure, so what has to change is the footing AREA, and area goes with the square of the dimension. Doubling the soil capacity does not halve the diameter, it divides it by the square root of two. This tool scales the table dimension by the square root of 1,500 divided by your psf, then rounds up to the next whole inch because you cannot buy a 23.4 in tube form. Most deck footing size charts online skip this entirely and just print the 1,500 psf row.
Corner posts, thickness and frost depth
A corner post carries roughly a quarter of what an interior post carries, and note 2 permits the tabulated plan dimension to be multiplied by 0.9 for corner posts. That reduction applies to the plan size, not to the thickness, so the pad stays as deep as the table says. Frost depth is separate again: it sets how far down the bottom of the footing has to sit, it comes from your local code rather than from this table, and the tool takes it as an input instead of guessing.
When this stops being a table lookup
DCA 6 is a prescriptive design guide for a specific, limited class of deck, and stepping outside it means an engineered design rather than a bigger number off the chart. Take it to a licensed engineer when the beam span or joist span exceeds the tabulated range, when the deck carries a hot tub, a planter, an outdoor kitchen or any other concentrated load, when the soil is fill, soft clay or otherwise questionable, when the footing sits on or near a slope, when the deck is more than one storey above grade, or when the building official asks for calculations. Nothing on this page is a stamped design, and a footing schedule is not a substitute for a soils report where one is required.
One table, transcribed verbatim, with its assumptions on the page
Most deck footing size calculators on the web will not tell you which guide they run, what soil they assume, or whether the corner reduction was applied. Here is ours in full.
Table 4, footing columns only
The dataset is the footing half of AWC DCA 6-2015 Table 4, page 12, transcribed verbatim rather than smoothed. The post-height half of that table is deliberately left out, because post height is IRC Table R507.4 territory and belongs to a different tool.
The assumptions are stated, not buried
Table 4 note 2 carries the whole basis: 1,500 psf assumed soil bearing, 150 pcf concrete, and permission to multiply the tabulated dimension by 0.9 for a corner post. The design loading behind DCA 6 is 40 psf live plus 10 psf dead, on No.2 grade lumber in wet service.
The progression is not linear
The tabulated sizes do not step evenly, and they were not rounded to make them look tidy. A 12 ft beam span at a 14 ft joist span reads 30 in round or 26 in square at 13 in thick, and the next row up is not simply one inch more.
24″
Round footing, 14 ft joist span, interior post, at the table's 1,500 psf.
30″
The same joist span and soil. Four feet of beam adds six inches of footing.
35″
Still the same joist span. This is where post spacing starts paying for itself.
37″
The widest beam span DCA 6 tabulates. Past this it is an engineered design.
All four are round footings at a 14 ft joist span on an interior post at 1,500 psf. Drop the joist span to 10 ft and the 12 ft beam reads 25 in instead of 30 in; take it to 18 ft and the same beam reads 34 in. Shortening a span by adding one post is almost always cheaper than the concrete it saves you from pouring.
Which guide applies, and who actually decides
This tool runs AWC DCA 6-2015. Your building department may enforce a different edition, may have adopted the IRC deck provisions in Section R507 instead, or may publish its own amended deck detail, and a locally adopted document beats anything on this page. IRC R104 and R106 give the building official the authority, and the AHJ is the final word on your permit and on what soil bearing value you are allowed to assume. A prescriptive footing table is a worst-case lookup, not an engineering calculation, and nothing here is a stamped design.
Sources
- American Wood Council, DCA 6-2015 Prescriptive Residential Wood Deck Construction Guide (PDF), Table 4 on page 12 for footing sizes and note 2 for the soil, concrete and corner-post assumptions.
- ICC Digital Codes — 2021 International Residential Code, Chapter 5, for the deck provisions in Section R507 and the presumptive soil bearing values your AHJ may apply instead.
Table transcribed 4 September 2026. Deliberately absent: no post height column for 6x6 posts, because that is IRC Table R507.4 and a different tool; no soil bearing value above 1,500 psf offered as a default, because assuming one without a report or an AHJ position is the most expensive mistake on this page; no frost depth table by state, because frost depth is set locally and no national table is authoritative; no hot tub, planter or concentrated-load cases, because those leave DCA 6 entirely.
Deck footing FAQs
How do I size footings when I do not know the soil bearing capacity?
DCA 6-2015 Table 4 assumes 1,500 psf, the IRC presumptive minimum for unknown soil, so that is the safe default. If your AHJ presumes a higher value or a soils report gives one, footings get smaller; on fill or soft clay they get larger or the AHJ requires a geotech. Never assume above 1,500 psf without something in writing.
What is the tributary area on a deck footing?
Half the beam span each way times half the joist span, for interior posts. That area times 50 psf, being 40 live plus 10 dead per DCA 6, gives the load the footing must spread. Corner posts carry roughly a quarter of an interior post's load, which is why one footing size across the whole deck wastes concrete.
How big should deck footings be for a 12 ft beam span?
At a 14 ft joist span on an interior post at 1,500 psf, DCA 6 Table 4 gives 30 in round or 26 in square, 13 in thick. Drop the joist span to 10 ft and it falls to 25 in round. Those are table values at the assumed soil; run your actual bearing capacity, because the size scales by the square root of 1,500 divided by your psf.
Does deck footing depth come from this table?
No. Table 4 gives the plan size and the thickness of the pad, not how deep it sits. Depth is set by your local frost line, which is a local code figure rather than a national one, and the footing bottom must reach below it. This tool takes your frost depth as an input and never assumes one.
Can I use a smaller footing under a corner post?
Yes, within limits. Table 4 note 2 permits the tabulated plan dimension to be multiplied by 0.9 for a corner post, because a corner carries far less tributary area than an interior post. That 0.9 applies to the plan size only, so the pad thickness stays as the table gives it, and the frost depth does not change either.
How much concrete for deck footings should I order?
Work it per footing and then total it, because corner and interior footings are different sizes. The tool returns the volume for each footing and for the whole deck from your footing count. Order with waste on top and remember the tabulated sizes assume 150 pcf concrete, which is ordinary normal-weight mix.
Sizing the footing is free. Booking the deck is the hard part
Which is why it matters that the homeowners asking you to price a deck are the ones who can fund footings, framing and a permit rather than shopping three quotes for the cheapest. Filtering for that before the site visit is a marketing problem, not a framing one.
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