Deck Footing Calculator
How big should footings be for a 12x16 deck?
For a 16 ft long, 12 ft deep deck attached with a ledger, one beam and posts 8 ft apart, at 50 psf (40 live + 10 dead) on 1,500 psf soil: 3 footings, the busiest carrying 48 sq ft or 2,400 lb. That needs 230 sq in of bearing, so an 18 in round or 16 × 16 in square footing. At 12 in thick, an 18 in round footing is 1.77 cu ft, or 3 bags of 80 lb concrete each.
This deck footing calculator estimates how many footings a deck needs and how big each one should be. Enter the deck length and depth in feet, whether it is attached to the house with a ledger or freestanding, the number of beams, the maximum post spacing, the design loads and the soil bearing capacity.
It returns the number of footings, the load on the busiest one, the minimum round or square footing size, and the concrete in 60 or 80 lb bags. Plan the decking and joists with the [deck calculator](/tools/deck-calculator).
Enter your details
How to Use the Deck Footing Calculator
- 1
Describe the deck
Enter the length along the house (the direction the beams run) and the depth out from the house in feet. Choose attached (ledger) or freestanding, and the number of beams.
- 2
Set post spacing
Enter the maximum post spacing along each beam in feet. Posts go at each beam end with the rest spread evenly.
- 3
Check loads and soil
The defaults are 40 psf live, 10 psf dead and 1,500 psf soil bearing. Change them only if your plans, code or a soil report say so.
- 4
Choose the concrete
Pick a round or square footing, the bag size and the footing thickness or pier depth in inches.
How the deck footing calculator works
Beams run along the deck length, spaced evenly across the depth, with the outer beam at the deck edge. Attached: the ledger supports the house side, so joist span = depth ÷ beams. Freestanding: beams sit at both edges, so joist span = depth ÷ (beams − 1).
Each beam carries the deck halfway to the next support on each side. An outer beam carries half a joist span; an interior beam carries a full one. Along the beam, posts are spaced evenly at or below your maximum spacing, with one at each end. An interior post carries one full spacing of beam; an end post carries half.
Every footing is sized for the busiest one: tributary area times the total design load gives pounds, and dividing by the soil bearing capacity gives the bearing area (times 144 for square inches). Diameter and square side are rounded up to the next whole inch, and concrete uses the rounded size.
Tributary area = post spacing × beam strip width
Load (lb) = tributary area × (live psf + dead psf)
Footing area (sq in) = load ÷ soil bearing psf × 144
Round diameter = √(4 × area ÷ π) · Square side = √area (both rounded up)
Concrete (cu ft) = footing area × thickness in ÷ 1,728 · Bags = cu ft ÷ 0.60 (80 lb) or 0.45 (60 lb)Worked example: a 12 x 16 ft attached deck
- Layout: 16 ft along the house, 12 ft deep, ledger plus one beam at the outer edge. Joist span = 12 ÷ 1 = 12 ft.
- The beam carries half the joist span: a 6 ft wide strip of deck. The ledger carries the other 6 ft.
- Posts at 8 ft maximum: 16 ÷ 8 = 2 spans, so 3 posts (one at each end, one in the middle), 8 ft apart.
- Busiest footing (middle post): 8 ft × 6 ft = 48 sq ft. Load = 48 × 50 psf = 2,400 lb.
- Bearing area: 2,400 ÷ 1,500 = 1.6 sq ft, × 144 = 230.4 sq in.
- Round: √(4 × 230.4 ÷ 3.1416) = 17.1 in, rounded up to 18 in. Square: √230.4 = 15.2 in, rounded up to 16 × 16 in.
- Concrete for an 18 in round footing 12 in thick: 3.1416 × 9² × 12 ÷ 1,728 = 1.77 cu ft, or 3 bags of 80 lb (4 of 60 lb). All 3 footings total 5.30 cu ft: 9 bags of 80 lb or 12 of 60 lb.
Footing size by load and soil
Minimum round diameter / square side in inches for the busiest footing, at 50 psf total load (40 live + 10 dead). Rounded up to the next whole inch.
| Tributary area | Load | 1,500 psf soil | 2,000 psf soil | 3,000 psf soil |
|---|---|---|---|---|
| 24 sq ft | 1,200 lb | 13 in / 11 in | 11 in / 10 in | 9 in / 8 in |
| 32 sq ft | 1,600 lb | 14 in / 13 in | 13 in / 11 in | 10 in / 9 in |
| 36 sq ft | 1,800 lb | 15 in / 14 in | 13 in / 12 in | 11 in / 10 in |
| 48 sq ft | 2,400 lb | 18 in / 16 in | 15 in / 14 in | 13 in / 11 in |
| 56 sq ft | 2,800 lb | 19 in / 17 in | 17 in / 15 in | 14 in / 12 in |
| 64 sq ft | 3,200 lb | 20 in / 18 in | 18 in / 16 in | 14 in / 13 in |
| 72 sq ft | 3,600 lb | 21 in / 19 in | 19 in / 17 in | 15 in / 14 in |
| 80 sq ft | 4,000 lb | 23 in / 20 in | 20 in / 17 in | 16 in / 14 in |
| 96 sq ft | 4,800 lb | 25 in / 22 in | 21 in / 19 in | 18 in / 16 in |
Footings for common deck sizes
Length is along the house and beams. Attached decks use a ledger plus one beam; freestanding decks use two beams. All at 50 psf on 1,500 psf soil, with round footings 12 in thick and 80 lb bags.
| Deck | Type | Max post spacing | Footings | Busiest load | Round / square | 80 lb bags each |
|---|---|---|---|---|---|---|
| 12 × 10 ft | Attached | 6 ft | 3 | 1,500 lb | 14 in / 12 in | 2 |
| 12 × 12 ft | Attached | 6 ft | 3 | 1,800 lb | 15 in / 14 in | 3 |
| 16 × 12 ft | Attached | 8 ft | 3 | 2,400 lb | 18 in / 16 in | 3 |
| 20 × 12 ft | Attached | 7 ft | 4 | 2,000 lb | 16 in / 14 in | 3 |
| 20 × 14 ft | Attached | 7 ft | 4 | 2,333 lb | 17 in / 15 in | 3 |
| 16 × 16 ft | Attached | 8 ft | 3 | 3,200 lb | 20 in / 18 in | 4 |
| 12 × 12 ft | Freestanding | 6 ft | 6 | 1,800 lb | 15 in / 14 in | 3 |
| 16 × 12 ft | Freestanding | 8 ft | 6 | 2,400 lb | 18 in / 16 in | 3 |
| 20 × 16 ft | Freestanding | 7 ft | 8 | 2,667 lb | 19 in / 16 in | 4 |
Concrete per round footing
Volume and bags for one round footing at 8 or 12 in thick, rounded up to whole bags. The concrete calculator handles deeper piers and tubes.
| Diameter | Thickness | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 12 in | 12 in | 0.79 | 2 | 2 |
| 14 in | 12 in | 1.07 | 2 | 3 |
| 16 in | 8 in | 0.93 | 2 | 3 |
| 16 in | 12 in | 1.40 | 3 | 4 |
| 18 in | 12 in | 1.77 | 3 | 4 |
| 20 in | 12 in | 2.18 | 4 | 5 |
| 24 in | 12 in | 3.14 | 6 | 7 |
Edge cases and assumptions
- Joist cantilever: if joists run past the outer beam, that beam carries more than half the joist span, and the footings need to be bigger. This calculator assumes the beam sits at the deck edge.
- Beam overhang: if the beam runs past the end posts, the end posts carry more. The calculator assumes posts at both beam ends.
- Hot tubs, planters and heavy snow need their own design; raising the live load input does not engineer a hot tub pad.
- Footing weight and shape: the footing's own weight is ignored, and concrete is the footing size times the thickness input. A pier on a wider footing, or a belled footing, needs a separate volume calculation.
- Stairs: a stair landing pad or stringer footing is extra. Lay out the steps with the stair calculator.
Code, frost line and when to call a pro
Treat these results as an estimate. Most US jurisdictions follow IRC Section R507 or a local deck guide for footings (including IRC Table R507.3.1), posts and beams, and local code can require larger footings than this math. The bottom of every footing must sit below your local frost line on undisturbed soil. Most decks need a permit and inspection, and some areas require footing holes to be inspected before the pour. Ask your building department for the frost depth and soil bearing value to use, and get a builder or engineer for raised, multi-level or heavily loaded decks.
Features
- Attached (ledger) or freestanding decks with 1 to 4 beams
- Number of posts and footings from your maximum post spacing
- Tributary area and load on the most heavily loaded footing
- Minimum footing area with round diameter and square size, rounded up
- Concrete per footing and total 60 or 80 lb bags
Frequently Asked Questions
How many footings does a deck need?
It depends on the number of beams and the post spacing. Each beam needs a post at each end plus enough in between to keep posts within the maximum spacing. A 16 ft beam with posts no more than 8 ft apart needs 3 posts. An attached deck with one beam has one row of footings; a freestanding deck has at least two.
How deep should deck footings be?
The bottom of each footing must be below your local frost line and on undisturbed soil. Frost depth varies widely across the US, from a few inches in warm areas to several feet in cold ones, so ask your local building department for the required depth.
What soil bearing capacity should I use?
Many builders use 1,500 psf when the soil type is unknown, which is the calculator default and the conservative choice. Higher values like 2,000 or 3,000 psf apply only to soils your local code or a soil report classifies that way. Using a higher value makes footings smaller, so do not guess.
How many bags of concrete for a deck footing?
An 18 inch round footing 12 inches thick holds about 1.77 cubic feet, which is 3 bags of 80 lb concrete or 4 bags of 60 lb. A 12 inch round footing 12 inches thick is about 0.79 cubic feet, or 2 bags of either size. Deeper piers need more.
What design load should I use for a deck?
A typical residential deck is designed for 40 psf live load plus 10 psf dead load, 50 psf in total, which are the calculator defaults. Areas with heavy snow, hot tubs or heavy materials like stone pavers need higher loads, set by your local code or an engineer.
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