Tributary area: how much deck each post carries
Every post carries a rectangle of deck, and the two sides of that rectangle come from different parts of the frame. Along the beam, the post picks up half the span to the post on its left and half the span to the post on its right - one full post spacing. Across the deck, it picks up half the joist span, because each joist is supported at both ends and splits its load between the ledger at the house and the beam.
So the tributary area is post spacing multiplied by half the joist span. A deck with posts at 8 ft and a 12 ft joist span gives 8 × 6 = 48 ft² per post. That is the number every other figure on this page is built from.
A cantilever changes it. Joists running past the beam hang their overhang weight on the beam, so the tributary depth becomes half the backspan plus the full cantilever. A 2 ft overhang on a 16 ft backspan gives 8 + 2 = 10 ft of tributary depth, not 8.
Multiply the area by the design load and you have pounds. The IRC sets 40 psf as the minimum live load for exterior decks in Table R301.5, and 10 psf is a conventional dead-load allowance for ordinary wood framing and decking, giving 50 psf in total - the same combination the DCA 6 footing tables are built on.
Finally, divide by the soil's allowable bearing pressure to get the area of footing that spreads the load safely. That area sets the diameter or the side. For the concrete volume once the size is fixed, the Sonotube column calculator handles the pier above the pad.
From pounds to a footing diameter
Soil bearing pressure is a stress: pounds per square foot the ground can take without settling more than the structure tolerates. Required area is therefore load divided by pressure, and nothing else. A 2,400 lb post on soil rated 1,500 psf needs 2,400 ÷ 1,500 = 1.6 ft² of footing.
Turning area into a dimension is geometry. A square footing has side √A, so 1.6 ft² gives √1.6 = 1.265 ft = 15.2 in. A round footing has area πd²/4, so d = √(4A/π) = √(6.4/π) = √2.0372 = 1.427 ft = 17.1 in. The round footing is always the larger dimension, because a circle is a less efficient way to enclose area than a square - about 12.8 percent more diameter than side for the same area.
Bearing values come from IRC Table R401.4.1 unless a soils report says otherwise: 1,500 psf for clay, sandy clay, silty clay and clayey silt; 2,000 psf for sand, silty sand, clayey sand and silty gravel; 3,000 psf for sandy gravel and gravel; higher only on rock. Those are presumptive values chosen to be safe without testing, and using a higher number without a report is the single fastest way to under-size every footing on the deck.
Concrete volume is area times thickness, which is why a bigger footing costs more twice over - wider and, usually, thicker. IRC R403.1 sets a 6 in minimum footing thickness, and DCA 6 tabulates greater thicknesses for larger footings so the pad can develop the bending it needs to spread the load.
Worked example: a 12 by 16 ft deck on four posts
A deck runs 12 ft out from the house and 16 ft along it, with a single beam at the outer edge carried on posts at 8 ft centres. The soil is a clayey silt, so 1,500 psf.
- Tributary depth. The joists span 12 ft from ledger to beam with no cantilever, so each post picks up 12 ÷ 2 = 6 ft.
- Tributary area. 8 ft post spacing × 6 ft = 48 ft².
- Load. 48 × (40 + 10) = 48 × 50 = 2,400 lb per post.
- Required bearing area. 2,400 ÷ 1,500 = 1.6 ft².
- Round footing. d = √(4 × 1.6 ÷ π) = √2.0372 = 1.4273 ft = 17.13 in, so pour an 18 in pad or set an 18 in fibre tube.
- Square footing. √1.6 = 1.2649 ft = 15.18 in, so a 16 in square pad.
- Concrete. At 8 in thick, 1.6 × 0.6667 = 1.067 ft³ per footing, which is two 80 lb bags. Across four posts that is 4.27 ft³, or 0.158 yd³ - bag territory.
Notice that the ledger carries the other half of the deck. Half the load never reaches a footing at all, which is why a ledger connection has to be correct: it is holding up as much deck as the beam is.
Reading the result, and where the number stops applying
Two limits sit below the calculated diameter. The first is practical: fibre tube forms start at 6 in and post bases need a pad wide enough to bolt to, so a footing under about 12 in diameter is rarely worth pouring even when the arithmetic allows it. The second is code: IRC R403.1 sets a minimum footing thickness of 6 in, and local amendments often set a minimum plan size as well. When the calculated diameter comes out below 12 in, the practical minimum governs and the calculator says so.
Above the calculated diameter sits a different limit. DCA 6 tabulates footings for ordinary residential decks; once the required size runs past roughly 3 ft, the load is high enough that a designed footing, a closer post spacing or a verified bearing value is the right answer rather than simply pouring a larger pad. The cheapest lever is almost always post spacing: halving it halves the load per footing, and two modest footings are usually cheaper than one large one.
Depth is a separate question from size. Footings must bear below the local frost line - a depth set by your jurisdiction, not by this calculation - and on undisturbed soil or properly compacted fill. A correctly sized footing bearing on 18 in of loose backfill will settle regardless of its area.
Nothing here checks the post, the beam or the ledger. Post size comes from the axial load and the unbraced height; beam size from the span and the same tributary width; and the ledger connection from the fastener schedule in IRC Table R507.9.1.3(1). The beam section modulus calculator and the floor joist span calculator cover the framing above.
Required round footing diameter, 50 psf total load
| Joist span | 6 ft posts, 1500 psf | 8 ft posts, 1500 psf | 10 ft posts, 1500 psf | 8 ft posts, 2000 psf | 8 ft posts, 3000 psf |
|---|---|---|---|---|---|
| 8 ft | 12.1 | 14.0 | 15.6 | 12.1 | 9.9 |
| 10 ft | 13.5 | 15.6 | 17.5 | 13.5 | 11.1 |
| 12 ft | 14.8 | 17.1 | 19.1 | 14.8 | 12.1 |
| 14 ft | 16.0 | 18.5 | 20.7 | 16.0 | 13.1 |
| 16 ft | 17.1 | 19.8 | 22.1 | 17.1 | 14.0 |
| 18 ft | 18.2 | 21.0 | 23.5 | 18.2 | 14.8 |
No cantilever. Values under 12 in are governed by practical minimums rather than bearing. Doubling the bearing pressure divides the required area by two and the diameter by √2, which is why the 3000 psf column is close to 70 percent of the 1500 psf column.
Mistakes that under-size a deck footing
- Using the full joist span instead of half. Each joist splits its load between the ledger and the beam. Using the whole span doubles the footing area you think you need - or, done the other way round, halves what you actually build.
- Forgetting the cantilever. An overhang past the beam adds its full length to the tributary depth, not half.
- Assuming a high bearing value. Without a soils report, IRC Table R401.4.1 caps you at 1,500 psf on clay and silt. Guessing 3,000 halves every footing on the deck.
- Ignoring corner and end posts with different geometry. A post at the end of a beam picks up only half a bay along the beam, and a post supporting a stair landing or a hot tub picks up far more. Size each condition separately.
- Bearing on backfill. The footing must sit on undisturbed soil or compacted structural fill, below the frost line. Area cannot compensate for a soft bearing surface.
- Neglecting concentrated loads. Hot tubs, planters, outdoor kitchens and heavy furniture are point loads well outside the 40 psf assumption, and they need their own posts and footings.
Bearing area is one of several checks
This calculator does the bearing check in AWC DCA 6 and the IRC. It does not size the post, the beam, the joists or the ledger connection, it does not check overturning or lateral load, and it does not check the footing's own thickness for punching shear or bending - DCA 6 tabulates minimum thicknesses that grow with footing size for exactly that reason. Where the deck is more than a storey above grade, carries a hot tub, or supports a roof, the load path needs a designer, not a table.
Footings, piers and the alternatives
Three details do the same job under a deck post, and the choice is usually about frost depth and site access. A poured pad with a separate pier on top gives the largest bearing area for the least concrete and is the standard where frost depth is shallow. A full-depth fibre tube pier with a flared or belled base carries bearing and depth in one pour and suits deep frost lines. A helical or driven pile is the answer where the soil is poor, the access is bad, or excavation is impractical, and its capacity is verified by installation torque rather than by a bearing table.
Whichever you use, the arithmetic above sets the area at the bottom. A tube pier with no wider base has only its own cross-section bearing on the soil - a 10 in tube offers 0.545 ft², which at 1,500 psf carries 818 lb, or about a third of the example post load. That is the most common failure in owner-built decks: a pier deep enough for frost, but far too narrow to bear.
The rest of the deck follows from the same layout. Beam and joist sizing come from the span tables and the floor joist span calculator; decking quantities from the deck board calculator; stairs down to grade from the stair rise and run calculator, and an accessible route alongside them from the wheelchair ramp slope calculator.