How Much Concrete for a Deck Footing? Bags & Cost

A standard 12-inch sonotube footing poured 4 feet deep takes about 3.1 cubic feet of concrete, close to 3.45 cubic feet once you add waste, which works out to 6 bags of 80-lb concrete mix. Smaller 8-inch tubes need as few as 3 bags, while a wide 24-inch footing under a heavy beam can take 23 bags or more. Below is the volume by tube size and depth, what that means in bags versus ready-mix, and a calculator that gives the exact count for your holes.

The formula: how much concrete one deck footing needs

A deck footing poured in a sonotube is a cylinder, so its volume is pi times the radius squared times the depth, all in feet. Unlike a fence post, the footing is usually poured solid with no post embedded, since the post base sits on top via a bracket after the concrete cures.

A 12-inch tube has a radius of 0.5 feet. At 4 feet deep that is pi times 0.25 times 4, about 3.14 cubic feet. Add 10% for spillage and an uneven hole bottom and you land at roughly 3.45 cubic feet, which rounds up to 6 bags of 80-lb concrete mix.

Volume (ft3) = π x radius2 x depthBags needed = (Volume x 1.10) ÷ bag yield

Every footing on a deck can use a different diameter, so plug each hole into the sonotube calculator rather than reusing one number for the whole job.

Concrete by sonotube diameter and depth

The table below assumes a 4-foot deep pour, a common depth in mixed and cold climates, with an 80-lb bag yielding 0.60 cubic feet after a 10% waste allowance. Deeper or shallower holes scale up or down from these numbers.

Tube diameter	Cubic feet (4 ft deep)	80 lb bags (+10%)
8 in	1.4	3
10 in	2.2	4
12 in	3.1	6
16 in	5.6	10
18 in	7.1	13
24 in	12.6	23

A 12-inch tube 3 feet deep needs about 5 bags, and the same tube at 5 feet needs about 8 bags. Depth changes the bag count almost as much as diameter does, so measure the actual hole before buying.

Picking the right diameter and depth for the footing

Post size alone does not determine tube diameter. Use the applicable code table and notes, approved plans or an engineered design based on tributary area, loads and soil.

Depth depends on the adopted frost-protection, embedment, soil and inspection requirements.

After diameter and depth are approved, the deck footing guide explains the quantity inputs.

Bags versus ready-mix: what deck footings cost

For a full deck project that also includes a landing pad or patio slab, the concrete calculator handles that flat work separately from the round footings.

Close-up of bagged concrete mix and a mixing tub beside a row of sonotube forms on a deck job site

What concrete mix and PSI to use

Use the concrete strength, exposure and placement method required by the adopted code and approved design. Product color or bag category does not establish compliance.

Use a fast-setting or conventional product only within its manufacturer instructions and approved application. Do not infer suitability from set time alone.

Ask the supplier for concrete that matches the project specification.

Avoiding common mistakes when pouring footings

The most common shortfall is skipping the waste factor. Hand-dug holes are never a perfect cylinder, and the last few inches of a tube always seem to eat more concrete than the math predicted. Round every bag count up, not down.

The second mistake is guessing depth instead of checking it. A footing poured 6 inches shy of the frost line can heave over a hard winter, cracking the bond between footing and post bracket. Confirm the frost depth with your local building department before you dig.

Consolidate the concrete as you pour by rodding or vibrating the tube, since trapped air pockets weaken the footing right where the post load concentrates.

By Calcnaut

Formula and declared-case checks run during the build; sources are listed for readers to review. Read our method and editorial policy.

Frequently asked questions

How many bags of concrete do I need for deck footings?

Use the approved footing geometry, subtract any displacement where applicable, and divide the concrete volume by the selected product's published yield. A 12-inch-diameter, 4-foot cylinder is about 3.14 cubic feet before allowance, which rounds to 6 bags at a 0.60-cubic-foot yield; this is a quantity example, not a footing recommendation.

Can I use Quikrete for deck footings?

Use a bagged product only if its current data sheet, mixing method and exposure properties satisfy the approved footing specification. Product category or advertised strength alone does not establish suitability for the loads, placement, curing and local requirements.

Is 4000 psi concrete good for deck footings?

Use the required strength and exposure class from the adopted code and approved design. A generic bag description does not prove that the selected product meets the project specification.

How much concrete for a 12x12 footer?

If 12x12 means a 12-inch-square pad 8 inches thick, its geometric volume is about 0.67 cubic feet and rounds to 2 bags at a 0.60-cubic-foot yield before project allowance. If it means a 12-inch tube, depth is also required. Neither example selects an adequate footing.

How deep should a deck footing be?

Use the frost-protection, embedment and bearing requirements in the adopted code and approved deck design. Obtain the local design depth and soil basis before entering dimensions; a climate range does not establish compliance for a project.

Sources and references

These references and structured source records support the method and material data used on this page.

How Much Concrete for a Deck Footing?

Key takeaways