How Much Concrete for a Deck Footing?
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.
Key takeaways
- A 12-inch sonotube footing 4 feet deep needs about 3.1 cubic feet of concrete, roughly 6 bags of 80-lb mix once waste is added.
- Tube diameter runs 8 to 24 inches depending on post load; depth is set by the local frost line, commonly 12 to 48 inches or more.
- One 80-lb bag of concrete yields 0.60 cubic feet, so it takes about 45 bags to fill a full cubic yard.
- Standard bagged concrete mix cures to about 4,000 psi, above the 2,500 to 3,000 psi most codes require for residential footings.
- Material for a single 12-inch footing runs roughly 50 to 70 dollars in 2026, including the tube form and bags.
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 yieldEvery 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
Tube diameter should be at least three times the post it supports. A 4x4 post (actual 3.5 inches) works on a 10-inch tube at minimum, while a 6x6 post (actual 5.5 inches) typically needs 12 inches or wider under real load.
Depth is a code question, not a preference. Footings must sit at least 12 inches below grade and below the local frost line, whichever is deeper, per IRC R507.3.1. Cold-climate frost lines commonly run 36 to 48 inches, while warm regions may allow 12 to 24 inches.
Both diameter and depth also depend on the tributary area each post carries, covered step by step in how to size deck footings, which walks through the load math before you dig.
Bags versus ready-mix: what deck footings cost in 2026
In 2026, an 80-lb bag of standard concrete mix runs about 8 to 9 dollars at most home centers, with sonotube forms adding 12 to 40 dollars depending on diameter and length. A single 12-inch footing 4 feet deep typically lands around 50 to 70 dollars in material, tube included.
Once a deck needs more than about 8 footings this size, the concrete adds up to roughly a full cubic yard, and ready-mix delivered by a mini-mix truck often beats hauling 40 or more bags by hand. Delivered ready-mix runs about 125 to 180 dollars per cubic yard, plus a short-load fee under the supplier minimum.
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.
What concrete mix and PSI to use
Standard bagged concrete mix, the gray bag sold as QUIKRETE or Sakrete Concrete Mix, cures to an average compressive strength of about 4,000 psi at 28 days. That is well above the 2,500 to 3,000 psi minimum most residential codes require for footings, so it is the default choice for deck piers.
Fast-setting mix, poured dry into the hole and soaked with water, works for lighter loads and reaches a workable set in 20 to 40 minutes. For structural deck footings carrying real weight, standard mixed-and-poured concrete packs tighter around the tube walls and is the safer default under inspection.
If your local mix supplier offers 3,000 psi ready-mix, that also clears most code minimums for footings, though 4,000 psi gives more margin against freeze-thaw cycling in cold climates.
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.
Frequently asked questions
How many bags of concrete do I need for deck footings?
Can I use Quikrete for deck footings?
Is 4000 psi concrete good for deck footings?
How much concrete for a 12x12 footer?
How deep should a deck footing be?
References
- QUIKRETE Concrete Mix (product data)
- IRC 2021 R507.3.1 Minimum Footing Size (ICC Digital Codes)
- AWC DCA-6 Prescriptive Residential Deck Construction Guide
- NRMCA: Ready Mixed Concrete