Deck Footing Size & Concrete
Enter the deck and beam layout; footing type, load, and soil are under the results.
Carpentry
Deck Footing Calculator
Quick answer: footings = beams × (deck length ÷ post spacing, rounded up, + 1). A 16 × 12 ft deck with one beam and posts 8 ft apart needs 3 footings. Each carries up to 64 ft² × 50 psf = 3,200 lb, so on 1,500 psf soil it needs about 2.13 ft² of bearing — a round base roughly 20 in across, before the footing's own weight.
Plan the footings under a deck: how many you need, how much load each one carries, the smallest base that suits your soil, and the concrete bags and sonotubes to buy for round piers, belled piers, or square pads.
Belled pier footing
How to calculate deck footings
Deck footing math has two separate questions. The first is geometry: how many footings the beam layout needs and how much concrete fills them. The second is structural: whether each footing's base is big enough to spread its share of the deck load over soil that can only resist so many pounds per square foot. This calculator answers both, but the structural part is a planning check, not a design — the permit drawings and your inspector have the final say.
Footings = beams × (⌈L ÷ spacing⌉ + 1) · Base area ≥ (Atrib × psf + footing weight) ÷ soil psf
Round base diameter = √(4 × area ÷ π) · square pad side = √area
- Lay out the beams. A ledger-attached deck usually has one beam near the outer edge; a freestanding deck has at least two. Each beam is one row of footings.
- Count posts per beam. Divide the beam length by the maximum post spacing allowed for your beam size, round up, and add one. The calculator then spaces posts evenly.
- Find the tributary area. The simple version is deck area ÷ footings. Because an interior post on a beam carries a full bay while end posts carry half, the calculator also finds the largest simple-span area (post spacing × the beam's share of the joist span) and uses whichever is larger.
- Convert to load. Multiply the tributary area by the design load — 50 psf is the 40 psf live load plus 10 psf dead load used for the IRC R507 deck tables — then add the weight of the footing concrete itself.
- Divide by soil bearing. Load ÷ allowable soil pressure is the minimum bearing area of the footing base. Compare it with the base you plan to pour: the bell diameter, the pad size, or the pier diameter for a straight pier.
- Total the concrete. Add cylinder, bell (a truncated cone), or pad volumes, multiply by the number of footings, add waste, and divide by bag yield.
Worked example
A 20 ft × 12 ft deck is ledger-attached with one beam. The beam size allows posts up to 7 ft apart. Each footing is a 24 in square pad, 10 in thick, with a 10 in sonotube pier 36 in tall on top. Soil is assumed at 1,500 psf, load at 50 psf, waste at 10%.
- Posts: 20 ÷ 7 = 2.86 → 3 spans, so 4 footings at 6.67 ft
- Tributary area: 240 ft² ÷ 4 = 60 ft² average; largest simple span 6 ft × 6.67 ft = 40 ft² → use 60 ft²
- Concrete per footing: pad 2 × 2 × 0.833 = 3.33 ft³ + pier π × 0.417² × 3 = 1.64 ft³ = 4.97 ft³ (745 lb)
- Load: 60 × 50 = 3,000 lb + 745 lb = 3,745 lb → 3,745 ÷ 1,500 = 2.50 ft² needed; a 24 in pad gives 4.0 ft², so it passes (minimum about 19 in square)
- Total: 4 × 4.97 × 1.10 = 21.87 ft³ = 0.81 yd³ → 37 bags of 80 lb mix and four 36 in pier tubes
Minimum round footing base by tributary area and soil
Bearing area only, at a 50 psf design load. The footing's own weight is not included here (the calculator adds it), so treat these diameters as lower bounds and compare them with IRC Table R507.3.1 and your local deck guide.
| Tributary area | Load at 50 psf | 1,500 psf soil | 2,000 psf soil | 2,500 psf soil | 3,000 psf soil |
|---|---|---|---|---|---|
| 20 ft² | 1,000 lb | 11.1 in | 9.6 in | 8.6 in | 7.8 in |
| 40 ft² | 2,000 lb | 15.6 in | 13.5 in | 12.1 in | 11.1 in |
| 60 ft² | 3,000 lb | 19.1 in | 16.6 in | 14.8 in | 13.5 in |
| 80 ft² | 4,000 lb | 22.1 in | 19.1 in | 17.1 in | 15.6 in |
| 100 ft² | 5,000 lb | 24.7 in | 21.4 in | 19.1 in | 17.5 in |
| 120 ft² | 6,000 lb | 27.1 in | 23.5 in | 21.0 in | 19.1 in |
Deck footing notes and common mistakes
- A 12 in sonotube alone is often too small on weak soil. Its base is only 0.79 ft². At 1,500 psf that supports about 1,180 lb, or roughly 23 ft² of deck at 50 psf. That is why many deck guides call for a bell or a wider pad under the tube.
- Interior posts carry more than end posts. On a beam with three posts, the middle one supports a full bay and the end posts half a bay each. Over a continuous beam the first interior post can take about 25% more than the simple-span share — one more reason to round footing sizes up.
- Snow can govern. Decks are designed for at least 40 psf live load, but where the ground snow load is higher, the deck tables and footing sizes change. Raise the design load to match your local requirement.
- Use presumptive soil values unless tested. IRC Table R401.4.1 lists 1,500 psf for clays and silts, 2,000 psf for sands and silty or clayey gravels, and 3,000 psf for sandy gravel. Fill, organic soil, and soft wet clay need an engineer.
- Get below the frost line. Footing bottoms generally must sit below the local frost depth and on undisturbed soil. The pier length in this calculator should reach from the footing base to the top of the concrete, including any part above grade.
- Hidden cantilevers change the math. If joists cantilever past the beam or the beam cantilevers past the end posts, the posts carry more than this simple tributary area. Use the span tables or an engineer for those layouts.
Deck footing FAQ
How many footings does a 12x16 deck need?
It depends on the beams and post spacing. A 12 × 16 ft deck attached to the house with a ledger and one beam with posts no more than 8 ft apart needs 16 ÷ 8 = 2 spans, so 3 footings. A freestanding 12 × 16 deck with two beams needs 6. The allowed post spacing comes from the beam size in IRC Table R507.5 or your local deck guide.
How big should deck footings be?
Footing bearing area must be at least the load on the footing divided by the soil's allowable bearing pressure. A footing supporting 64 sq ft of deck at 50 psf carries 3,200 lb; on 1,500 psf soil it needs 2.13 sq ft, a round base about 20 inches in diameter, before adding the footing's own weight. Confirm against IRC Table R507.3.1 and your building department.
How deep do deck footings need to be?
Deck footings generally must extend below the local frost line, which ranges from about 12 inches in warm southern areas to 48 inches or more in northern states. The IRC allows limited exceptions for some low, freestanding decks. Your building department publishes the required frost depth and any minimum footing thickness.
How many bags of concrete do I need for a deck footing?
A 12 inch diameter pier 48 inches long holds 3.14 cubic feet, about 5.2 bags of 80 lb mix, so buy 6 per footing. Adding a 24 inch bell base 12 inches tall raises it to about 4.19 cubic feet, or 7 bags of 80 lb concrete per footing.
What soil bearing capacity should I use for deck footings?
When the soil has not been tested, deck designs commonly assume 1,500 psf, the lowest presumptive value in IRC Table R401.4.1, which applies to clays and silts. That table allows 2,000 psf for sands and silty or clayey gravels and 3,000 psf for sandy gravel and gravel. Use a higher value only if your building official accepts it.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.