How many deck footings do I need?
Count the beams, then the posts along each beam. A 16 × 12 ft deck attached to the house with a ledger needs one beam at the outer edge; with posts no more than 8 ft apart, that is 3 posts. Three 12 in × 36 in footings hold 7.07 cu ft of concrete, or 12 bags of 80 lb mix.
Worked example: a 16 × 12 ft attached deck
- Beams: 12 ft width ÷ 12 ft joist span = 1 beam at the outer edge. The house side is carried by the ledger.
- Posts per beam: 16 ft ÷ 8 ft = 2 spaces, so 3 posts.
- One footing: 3.14 × 0.5 ft × 0.5 ft × 3 ft = 2.36 cu ft.
- Three footings: 7.07 cu ft.
- Bags: 7.07 ÷ 0.6 = 12 bags of 80 lb concrete, or 16 bags of 60 lb.
Posts and concrete by deck size
With a 12 ft maximum joist span, posts no more than 8 ft apart and 12 in × 36 in footings.
| Deck | Type | Beams | Posts | 80 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | Attached | 1 | 3 | 12 |
| 12 × 12 ft | Attached | 1 | 3 | 12 |
| 16 × 12 ft | Attached | 1 | 3 | 12 |
| 20 × 12 ft | Attached | 1 | 4 | 16 |
| 16 × 16 ft | Attached | 2 | 6 | 24 |
| 12 × 12 ft | Freestanding | 2 | 6 | 24 |
| 16 × 12 ft | Freestanding | 2 | 6 | 24 |
Concrete per footing
| Footing size | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 in × 24 in | 0.7 cu ft | 2 | 2 |
| 10 in × 36 in | 1.64 cu ft | 3 | 4 |
| 12 in × 36 in | 2.36 cu ft | 4 | 6 |
| 12 in × 42 in | 2.75 cu ft | 5 | 7 |
| 16 in × 42 in | 4.89 cu ft | 9 | 11 |
| 16 in × 48 in | 5.59 cu ft | 10 | 13 |
An 80 lb bag of concrete mix yields about 0.6 cu ft and a 60 lb bag about 0.45 cu ft. Footings with a wider spread base or a tube form above grade need more; add the extra volume or a bag per footing.
Before you dig
- Footings must go below the frost line where you live. Your building department can tell you the required depth.
- Allowed post spacing, beam size and joist span depend on the lumber species, grade and size. The American Wood Council’s DCA 6 deck guide has prescriptive span tables based on the IRC.
- Most decks attached to a house need a permit and an inspection of the footing holes before concrete is poured.
- Call 811 before digging so buried utility lines are marked.
How the calculator works
Beams = deck width ÷ joist span, rounded up (+1 for a freestanding deck)
Posts per beam = deck length ÷ post spacing, rounded up, + 1
Concrete per footing = 3.14 × radius² × depth
Bags = total concrete ÷ bag yield, rounded up
Frequently asked questions
How far apart should deck posts be?
Often 6 to 8 ft, but the allowed spacing depends on the beam size and the joist span it carries. Use your plan or the span tables in the AWC DCA 6 guide.
How many bags of concrete per deck post?
A 12 in × 36 in footing holds about 2.36 cu ft, which is 4 bags of 80 lb mix or 6 bags of 60 lb mix.
How deep should deck footings be?
Below the local frost line, which ranges from a few inches in the South to 4 ft or more in northern states. Ask your building department.
Does a freestanding deck need more posts?
Yes. Without a ledger on the house, the deck needs a beam and posts on the house side too, so it usually has one more row of posts.
Sources
- STRUCTURE magazine, Residential Wood Deck Design
- JLC, Highlights from the American Wood Council’s DCA 6 deck guide
- Quikrete, Concrete Mix specification sheet
Guides for this calculator
- How many bags of concrete per fence post?Bags of fast-setting and regular concrete per post for 4×4 and 6×6 posts, with the hole size rules.