Deck Footing Calculator
Short answer: a 12 × 16 ft deck with footings every 8 ft needs 6 footings. At 12 inches wide and 4 ft deep, that is 0.70 cubic yards — about 35 × 80 lb bags with 10% waste.
Calculate your deck footings
How the calculation works
Footings are laid out on a grid: count along each side = deck size ÷ spacing, rounded down, plus one, then multiply the two counts. Each footing is a cylinder: volume = π × (diameter ÷ 2)² × depth. Multiply by the footing count, then divide cubic feet by 27 for cubic yards.
Worked example: a 12 × 16 ft deck at 8 ft spacing = (1 + 1) × (2 + 1) = 6 footings. One 12 in footing 4 ft deep = 3.14 cu ft = 0.12 cubic yards. Six of them = 0.70 cubic yards net (0.77 with 10% waste).
Concrete per footing (4 ft deep, net)
| Footing diameter | Volume | 80 lb bags |
|---|---|---|
| 10 in | 0.08 cubic yards | 4 |
| 12 in | 0.12 cubic yards | 6 |
| 16 in | 0.21 cubic yards | 10 |
| 18 in | 0.26 cubic yards | 12 |
Volumes are net (before waste). Bag counts use the 0.60 cu ft yield per 80 lb bag, rounded up.
Frequently asked questions
How many footings for a 12x16 deck?
At 8 ft spacing, 6 footings: 2 along the 12 ft side and 3 along the 16 ft side, from the spacing grid.
How do you calculate concrete for a deck footing?
Each footing is a cylinder: volume = π × (diameter ÷ 2)² × depth. Multiply by the footing count, then divide cubic feet by 27 for cubic yards.
How deep should deck footings be?
Below your local frost depth, following your deck plans and local code. Depth varies by climate — check both before digging.