Deck Footing Size Calculator: Tributary Area & Soil Bearing
Calculate deck footing size from post tributary area, design load, and soil bearing capacity, using the same 40 psf live load and 10 psf dead load assumptions behind IRC 2021 Table R507.3.1 and AWC DCA 6 Table B3. Enter beam spans and post spacing to find tributary area automatically, or enter a known area directly. Check post sizing separately with the deck post size chart.
🏘 Deck Footing Size Calculator
Tributary Area | 40/10 PSF Design Load | Round or Square Footings
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View Chart →Find Your Deck Footing Size by Tributary Area
Look up an approximate round footing diameter directly by tributary area and soil bearing value, at the standard 50 psf design load (40 psf live, 10 psf dead). Values are rounded to the nearest whole inch and consistent with AWC DCA 6 Table B3 methodology.
| Tributary Area (sq ft) | 1,500 psf | 2,000 psf | 3,000 psf | 4,000 psf |
|---|---|---|---|---|
| 20 | 12 in | 11 in | 9 in | 8 in |
| 40 | 17 in | 15 in | 13 in | 11 in |
| 60 | 21 in | 18 in | 15 in | 13 in |
| 80 | 24 in | 21 in | 17 in | 15 in |
| 100 | 27 in | 23 in | 19 in | 17 in |
| 120 | 29 in | 25 in | 21 in | 18 in |
| 140 | 31 in | 27 in | 22 in | 20 in |
| 160 | 34 in | 29 in | 24 in | 21 in |
Source: IRC 2021 Table R507.3.1 and AWC DCA 6 Table B3, at 50 psf total design load (40 psf live + 10 psf dead). This calculator's exact math method may produce slightly different results than a specific published table due to rounding conventions; always confirm against your adopted local code table for permit submissions.
How Footing Size Is Determined
Find Tributary Area
Multiply half the distance to each adjacent beam line by half the distance to each adjacent post to get the area one footing must support.
Apply Design Load
Multiply tributary area by the design load, typically 50 psf (40 live plus 10 dead), or by dead load plus snow load if snow governs.
Divide by Soil Capacity
Divide the total load by allowable soil bearing pressure to get required footing area, then convert to a round or square dimension.
Check Frost Depth
Compare the required footing width against your local frost depth requirement, since footings must extend below frost regardless of width.
Why Interior Posts Usually Need the Biggest Footings
A corner post on a typical deck picks up load from only one beam direction and one post spacing direction, often with an overhang reducing the load further on one side. An interior post, by contrast, sits between two beam lines and two adjacent posts, so it collects tributary area from all four directions. This is why interior footings are frequently the largest on a deck, even though corner footings might seem more exposed.
This calculator asks for beam span and post spacing on both sides of the post being checked specifically so it can handle any post position: enter 0 for a beam span or post spacing on any side where there is no adjacent beam or post, such as the ledger side of a corner post or the outer edge of an end post.
Why 50 psf Is the Standard Deck Design Load
The 40 psf live load reflects typical residential occupancy and furniture loading for an open-air deck, the same value used for most habitable residential floor areas. The additional 10 psf dead load accounts for the deck's own framing, decking material, and railings. Both values come from the assumptions built into AWC DCA 6 and referenced in IRC 2021 Table R507.3.1, and departing from them (heavier decking, hot tubs, unusual snow loads) requires a specific engineered calculation rather than the standard table.
💡 Tip - Size Every Footing to the Worst Case
Many deck builders calculate tributary area for the most heavily loaded post only, then use that single footing size for every post on the deck. This adds a small amount of extra concrete for lightly loaded posts but eliminates the risk of installing an undersized footing at the wrong location during construction.
Example Scenario: Attached Deck, Interior Post
Deck Layout
Ledger-attached deck. Post sits between the ledger (6 ft away) and an outer beam (8 ft further), with posts spaced 8 ft apart along the beam and a 2 ft overhang past this post.
Tributary area = 7 x 6 = 42 sq ft
This matches a commonly referenced deck footing worked example using the same tributary area geometry method.
Applying the Design Load
Standard 40 psf live + 10 psf dead = 50 psf design load. No snow load governing.
This is the total weight the footing beneath this post must transfer into the soil.
Sizing the Footing
Soil bearing capacity: 3,000 psf (sandy gravel)
Diameter = 2√(0.70/π) = 11.3 in exact, round up to a practical 12 in sonotube
On weaker 1,500 psf clay soil, this same post would need roughly a 16 in footing instead, illustrating how much soil strength affects the result.
Common Deck Footing Sizing Mistakes
⚠ Errors That Change the Result
- Calculating tributary area for a corner post only: Corner and end posts typically carry less tributary area than interior posts. Sizing every footing based on a corner post's smaller load can undersize interior footings.
- Forgetting to check snow load: In areas where ground snow load exceeds 40 psf, DCA 6 methodology uses dead load plus snow load instead of dead load plus the standard 40 psf live load, which can meaningfully increase required footing size.
- Ignoring frost depth in favor of the load calculation alone: A footing can be perfectly sized for load and still fail if it does not extend below the local frost line, since frost heave can lift an undersized-depth footing regardless of its diameter.
- Using a generic soil assumption without checking the actual site: Defaulting to 1,500 psf when the actual soil is sandy gravel at 3,000 psf produces an oversized, more expensive footing. Confirm soil type or get a test if the difference matters for the project budget.
- Mixing round and square dimension formulas: A required area translates to different dimensions for round versus square footings; using a square footing's side length as if it were a round footing's diameter (or vice versa) produces an undersized footing.
Using Footing Size Results for Deck Permits
Most US jurisdictions require a deck permit application to show footing size, depth, and spacing on a site plan, along with post and beam sizing. Use the deck beam span chart and deck joist span chart alongside this calculator to confirm the framing dimensions that feed into your tributary area inputs, and the deck post spacing chart to plan post layout before finalizing footing locations.
Once footing size is set, use the concrete deck footing calculator or post hole concrete calculator to estimate concrete volume and bag count. For decks outside conventional IRC/DCA 6 prescriptive limits, including elevated decks, hot tub loading, or unusual soil conditions, an engineered design reviewed by a licensed structural engineer is required per IBC 2024 Section 1604.
Frequently Asked Questions
Tributary area is the portion of deck surface whose weight transfers down through a single post to a single footing, found by multiplying half the distance to each adjacent beam or ledger by half the distance to each adjacent post. An interior post on a typical deck usually carries the largest tributary area and therefore requires the largest footing.
Per AWC DCA 6 and IRC 2021 Table R507.3.1, standard deck footing tables assume a 40 psf live load plus a 10 psf dead load, for a total design load of 50 psf, in areas where ground snow load is 40 psf or less. If ground snow load exceeds 40 psf, the higher of live load or snow load is used instead of live load, per the same methodology footingpad.com and other deck guides apply.
On the IRC default 1,500 psf soil, a post with 40 square feet of tributary area needs approximately a 17 inch round footing, and a post with 80 square feet needs approximately a 24 inch round footing, per IRC 2021 Table R507.3.1 and AWC DCA 6 Table B3. Stronger soil, such as 3,000 psf gravel, reduces these diameters by roughly 25 to 30 percent for the same tributary area.
Deck footings must extend below the local frost depth to prevent frost heave, which varies significantly by climate zone, from as little as 12 inches in the Gulf Coast to 48 inches or more in the northern Midwest and New England. IRC 2021 Section R403.1.4 requires footings to bear on undisturbed soil below the frost line, regardless of what the tributary area and soil bearing calculation determines for footing width.
No, but many builders size every footing on a deck to match the largest required size for simplicity and to reduce the chance of a field error. Sizing each footing individually based on its own tributary area saves some concrete but requires tracking which footing goes where during construction, and any layout change can invalidate a footing-by-footing plan.
Yes. IRC Table R401.4.1 sets 1,500 psf as the presumptive value for clay when no soil test has been performed, but sand, gravel, and other stronger soils have higher presumptive values under the same table, and a geotechnical report can confirm an even higher site-specific value. A smaller footing on stronger soil is code-compliant as long as it meets the same 50 psf design load requirement.
No. This calculator estimates footing size from tributary area and soil bearing capacity using standard prescriptive deck design assumptions. It does not verify beam or joist sizing, connection hardware, guardrail loading, or any condition outside conventional IRC/DCA 6 prescriptive limits, all of which may require an engineer for non-standard decks or heavier loading per IBC 2024 Section 1604.
Sources and Methodology
- IRC 2021, Table R507.3.1 (Minimum Footing Size for Decks) and Section R403.1.4 (Frost Protection).
- IRC 2021, Table R401.4.1 (Presumptive Load-Bearing Values of Foundation Materials).
- American Wood Council, DCA 6, "Prescriptive Residential Wood Deck Construction Guide," Table B3 (Footing Sizes Based on Tributary Area).
- Tributary area calculation method for deck posts: consistent with published deck construction guides including footingpad.com's isolated footing sizing methodology.
- Design load assumptions (40 psf live, 10 psf dead, snow load substitution above 40 psf): AWC DCA 6 and IRC 2021 Table R507.3.1 footnotes.
- IBC 2024, Section 1604 (General Design Requirements, licensed design professional review).
Last reviewed: September 2026. Reviewed by site author.
Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 Section 1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
Built by Muhammad Ramzan Babar, physics researcher (PhD candidate). Reviewed by site author.
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