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Cubic yards and bag count for deck footings, in any diameter and depth.
With these inputs:
| Result | Value |
|---|---|
| Concrete needed | 0.74 cubic yards |
| 80 lb bags | 34 |
| 60 lb bags | 45 |
| Per footing | 3.01 cu ft |
| Footings | 6 × 12" diameter |
| Total height poured | 46" (42" below grade + 4" above) |
| Volume per tube | 3.01 cu ft |
| Raw volume | 18.06 cu ft |
| With 10% waste | 19.87 cu ft |
| Tube form needed | 6 × 12" × 4 ft |
| Cheaper option | Bags from the store |
At 0.74 cubic yards, bags are the practical choice — 34 eighty-pound bags. A single mixer rental for the day beats a truck's short-load fee at this volume. The depth figure must reach below your local frost line, which is set by your building department and is not negotiable: a footing that stops short will lift every winter and take the deck with it. Volumes include a 10 percent waste allowance for spillage and over-dig.
Concrete is sold by the bag at the store and by the yard from a truck, and deck footings sit awkwardly between the two. This works out the exact volume for your footing size and count, converts it to both 60 and 80 pound bags with a waste allowance, and tells you the point where ready-mix becomes the cheaper option.
A 12 inch diameter footing poured 42 inches below grade and 4 inches above holds about 3.0 cubic feet. An 80 pound bag yields 0.60 cubic feet, so that is five bags per footing, or six with a waste allowance. Six footings of that size come to roughly 34 eighty-pound bags — around 2,700 pounds of material.
Below the local frost line, whatever that is where you live — 12 inches in parts of the south, 42 to 48 inches across much of the north, deeper in the coldest regions. Your building department publishes the figure and will not accept less. A footing that stops above the frost line gets pushed up every time the ground freezes, and the deck racks or pulls away from the house.
Bags win under about one and a half cubic yards, which is roughly six to eight standard footings. Past that, the labour of mixing and the cost per cubic foot both turn against you, and a short-load delivery becomes competitive even with the minimum charge and delivery fee. Get a quote — short-load pricing varies a lot by supplier.
It depends on your soil and the load each post carries. A bell-shaped footing form or a poured pad spreads the load over a wider area, which matters in soft or clay soils and under posts carrying a beam or a roof. On firm soil with a modest deck, a straight tube to the correct depth is usually accepted. Your inspector decides.
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