How Much Concrete for Deck Footings?
Six footings, 26 cubic feet, and the point where ready-mix starts to win.
Six 24×24×12 inch deck footings need 26.4 cubic feet of concrete — that is 44 bags of 80 lb, or one cubic yard of ready-mix. At this size ready-mix and bags cost about the same, because the supplier's one-yard minimum happens to line up with what you need.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Concrete needed by footing size and count
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| 18 x 18 x 12, one footing | 2.5 cu ft | 0.1 cu yd | 5 × 80 lb | Bags |
| 24 x 24 x 12, one footing | 4.4 cu ft | 0.2 cu yd | 8 × 80 lb | Bags |
| 24 x 24 x 12, six footings | 26.4 cu ft | 1 cu yd | 44 × 80 lb | Ready-mix |
| 24 x 24 x 12, twelve footings | 52.8 cu ft | 2 cu yd | 88 × 80 lb | Ready-mix |
| Strip footing, 10 ft x 12" x 8" | 7.3 cu ft | 0.3 cu yd | 13 × 80 lb | Bags |
| Strip footing, 30 ft x 16" x 12" | 44 cu ft | 1.6 cu yd | 74 × 80 lb | Ready-mix |
Footing length is in feet, width and depth in inches. 10 % waste. Ready-mix is billed with a one-yard minimum.
Footing size is set by the load, not by preference
A footing spreads the weight of the deck over enough ground that the soil can carry it. Getting this wrong is how decks sink, and it is not something to guess.
The two variables are the load the deck puts on each footing and the bearing capacity of your soil.
- Load. A deck is designed for around 40 to 50 pounds per square foot of live load plus its own dead load. A 12x16 deck with four footings puts roughly 2,500 to 3,000 lb on each one.
- Bearing capacity. Good compacted gravel or sand might carry 3,000 pounds per square foot. Soft clay might carry 1,500 or less.
A 24×24 inch footing has 4 square feet of bearing area, so on decent soil it can carry around 12,000 lb — far more than needed. That is why footing sizes for typical decks cluster around 18 to 24 inches: they are sized by the frost depth and the practical need to dig a hole, not by crushing the soil.
If you are in a cold climate, depth is the governing dimension. The footing must extend below the frost line, and that may be 36 to 48 inches. Check with your local building department — deck footings almost always need a permit and an inspection.
The one-yard crossover, live
Six footings is a good illustration of where bags stop being the obvious answer.
- 44 bags of 80 lb at $6 each is $264, and that is 3,520 lb of material to handle.
- One cubic yard of ready-mix at $165 is $165 — and your six footings need 0.98 yards, so you are paying for exactly one.
Ready-mix wins on both price and effort. The catch is access: a truck needs to get within chute reach, roughly 15 to 20 feet of the furthest footing, and it will not wait while you think about it.
On a tight site, bags are the only option and the extra cost is the price of access. Plan the pour so you can work continuously.
Frequently asked questions
How much concrete for 6 deck footings?
About 26.4 cubic feet, which is 44 bags of 80 lb mix or one cubic yard of ready-mix, for 24x24x12 inch footings with 10 % waste.
How big should deck footings be?
For most decks, 18 to 24 inches across and 12 inches deep, but the governing dimension in cold climates is depth — the footing must sit below the frost line, which can be 36 to 48 inches.
Do deck footings need a permit?
Usually yes, and they are inspected before the concrete goes in. Check with your local building department, because the requirements change by area and by frost depth.
Should I use ready-mix or bags for deck footings?
Six footings is right at the crossover. Ready-mix is cheaper and far less work if a truck can reach, but bags are the only practical option on a restricted site.