What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Beam length: 16 ft.
- Maximum post spacing: 8 ft.
- Number of beams: 1.
- Footing diameter: 12 in.
- Footing depth: 36 in. Must reach below your local frost depth and meet code.
- Extra for waste: 5 %.
- Concrete bag size: 80 lb.
What happens, step by step
- Posts at most 8 ft apart on a 16 ft beam: 3 footings each.
- Each 12 in wide, 36 in deep hole fills; the pallet counts bags.
- Posts stand on the footings, 8 ft apart.
- The beam sits on the posts and carries the deck to the footings.
The answer: footings (posts) is 3.
How it’s calculated
- Posts per beam = beam length ÷ max spacing, rounded up, + 1
- Concrete = π ÷ 4 × diameter² × depth × footings × (1 + waste %)
Worked example
One 16 ft beam, posts at most 8 ft apart, 12 in × 36 in footings, no waste.
Inputs
- Beam length
- 16 ft
- Maximum post spacing
- 8 ft
- Number of beams
- 1
- Footing diameter
- 12 in
- Footing depth
- 36 in
- Extra for waste
- 0 %
- Concrete bag size
- 80 lb
Results
- Footings (posts)
- 3
- Posts per beam
- 3
- Concrete volume
- 7.07 cu ft
- Bags of concrete
- 12 bags
Assumptions and limits
- Posts sit on top of the footing on a post base (post volume not subtracted).
- Allowable post spacing depends on beam size and load — it is an input here, not a check.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How deep should deck footings be?
Footings must bear below the frost line and on undisturbed soil. Frost depth ranges from a few inches to over 4 ft in the US — your building department sets the requirement.
How far apart can deck posts be?
It depends on beam size, species and joist span. Many code span tables fall in the 6–10 ft range for typical beams; check the table for your exact beam.