Concrete for Footings and Piers: Volume, Bags, and Tube Forms
October 21, 2026
Footings spread the weight of a structure over enough soil to carry it, and piers or posts support decks, porches, and sheds. The concrete you need is a volume calculation: strip footings use length × width × depth, while round piers use π × radius² × depth. A continuous footing 24 inches wide and 12 inches deep around a 120-foot perimeter contains 2 × 1 × 120 = 240 cubic feet, or 8.9 cubic yards.
The size and depth of a footing is not a free choice. It depends on soil bearing capacity, the loads above, and the local frost depth. Local building codes and often an engineer's drawing determine the dimensions; the formulas here help you order materials.
Skip the math and get your answer instantly:
Open the Concrete Calculator →Strip Footings
Multiply width (ft) × depth (ft) × total length (ft). A 24 × 12 inch footing is 2 ft × 1 ft in cross section, or 2 square feet. Over 120 feet that is 240 cubic feet. Dividing by 27 gives 8.89 cubic yards. Add 10% for uneven trench bottoms and spillage, and order about 9.8 cubic yards.
Include intersections and corners in your length measurement by measuring along the centerline of the footing and adding for overlaps. For a rectangle, add the four sides and subtract 4 times the footing width if you measured outer dimensions.
Round Piers and Post Holes
A round pier 12 inches in diameter and 4 feet deep has a radius of 0.5 feet, giving π × 0.25 × 4 = 3.14 cubic feet, about 0.116 cubic yards. Thirty such piers need 3.49 cubic yards. With 60-pound bags yielding about 0.45 cubic feet each, one pier takes roughly 7 bags, so 30 piers take about 210 bags.
Fiber tube forms hold the concrete in the upper part of the hole and above grade. Bell-shaped bottoms increase bearing area but need extra concrete.
| Pier diameter | Depth | Volume each | 60-lb bags each (0.45 cu ft) |
|---|---|---|---|
| 8 in | 36 in | 1.05 cu ft | 2.3 |
| 10 in | 42 in | 1.91 cu ft | 4.2 |
| 12 in | 48 in | 3.14 cu ft | 7.0 |
| 16 in | 48 in | 5.59 cu ft | 12.4 |
Frost Depth and Footing Size
In cold climates, the soil freezes to a depth known as the frost line, which varies from near zero in warm regions to 4 feet or more in northern ones. Footings must extend below it so that frost heave does not lift the structure. Local codes list the required depth. Digging deeper increases concrete volume proportionally.
The width must be enough to spread the load. A typical rule for light residential structures is a footing width of about twice the wall thickness, with a depth at least equal to the projection from the wall, but the engineering depends on soil. When in doubt, ask the building department or an engineer.
Ordering and Placing
Bags are practical for a few piers. For strip footings above about 2 yards, a ready-mix truck is more efficient, and a pump or chute can reach the trench. Pour concrete within the delivery time window, and keep it vibrated or rodded to eliminate voids. Place rebar as drawn on the plans, with specified clearance and cover so that steel is protected from moisture.
Cost and Time Planning
A 9-yard footing pour can be completed in a few hours by a small crew once forms and trenches are ready. Cure the concrete for several days before loading it, keep it moist in hot or dry weather, and protect it from freezing. Plan inspections before the pour, since many jurisdictions require them.
Frequently Asked Questions
How do I calculate concrete for a footing?
Multiply width × depth × length in feet to get cubic feet, then divide by 27 for cubic yards.
How much concrete is in a 12-inch by 48-inch pier?
About 3.14 cubic feet, or roughly 7 sixty-pound bags.
How deep should footings be?
Below the local frost line and as required by the engineer or building code.
How much extra should I order?
About 10% to cover irregular trench bottoms and waste.
Can I pour a footing with bagged concrete?
For small jobs yes. Large footings are more reliable with ready-mix to avoid cold joints.