Total Concrete
0.45 cu yd
Per Footing
2.75 cu ft
80lb Bags
21
Est. Cost
$116
Common: 8", 10", 12", 16", 18", 24"
Must be below frost line (typically 36–48")
Recommended 10% for typical post footings
2.75 cu ft
0.45 cu yd
$116
1,680 lbs
| Size | Cu Ft | Use |
|---|---|---|
| 8" × 42" | 1.22 | Light posts |
| 10" × 42" | 1.91 | Fence posts |
| 12" × 42" | 2.75 | Deck posts |
| 12" × 48" | 3.14 | Cold climate |
| 16" × 48" | 5.59 | Heavy loads |
| 18" × 48" | 7.07 | Large decks |
| 24" × 48" | 12.57 | Structural |
Inputs
Result
Per footing: π × (0.5)² × 3.5 = 2.749 cu ft. Raw total: 4 × 2.749 = 11.00 cu ft. With 10% waste: 12.10 cu ft = 0.45 cu yd. Bags: 12.10 / 0.60 = 20.2 → 21 bags of 80lb at $5.50 = $115.50.
Inputs
Result
Per footing: π × (0.5)² × 4.0 = 3.142 cu ft. Raw total: 6 × 3.142 = 18.85 cu ft. With 10% waste: 20.74 cu ft = 0.77 cu yd. Bags: 20.74 / 0.60 = 34.6 → 35 bags of 80lb at $5.50 = $192.50.
Inputs
Result
Per footing: π × (8/12)² × 4.0 = 5.585 cu ft. Raw total: 8 × 5.585 = 44.68 cu ft. With 10% waste: 49.15 cu ft = 1.82 cu yd. Bags: 49.15 / 0.60 = 81.9 → 82 bags of 80lb at $5.50 = $451.00.
Use the cylinder volume formula: V = π × (diameter/2)² × depth. Convert the result to cubic feet by ensuring diameter and depth are in feet (divide inches by 12). Then divide cubic feet by 27 to get cubic yards, or divide by 0.6 to get 80lb bags.
| Footing Size | Cu Ft Each | 80lb Bags (1 footing) | Typical Use |
|---|---|---|---|
| 8" × 42" | 1.22 | 3 bags | Fence posts, mailbox |
| 10" × 42" | 1.91 | 4 bags | Light deck posts |
| 12" × 42" | 2.75 | 5 bags | Standard deck posts |
| 12" × 48" | 3.14 | 6 bags | Cold climate deck |
| 16" × 48" | 5.59 | 10 bags | Heavy load posts |
| 18" × 48" | 7.07 | 12 bags | Large decks/pergolas |
| 24" × 48" | 12.57 | 21 bags | Structural columns |
Deck footings must extend below the frost line, which varies by location: 12" in the southern US, 36–48" in the Midwest and Northeast, and up to 60" in northern Minnesota and Alaska. Check your local building code for the exact frost depth requirement.
| Region | Frost Line | Min Footing Depth | Typical Sonotube |
|---|---|---|---|
| Southeast US | 6–12" | 12–18" | 12" × 24" |
| Mid-Atlantic | 18–24" | 24–30" | 12" × 36" |
| Midwest | 30–42" | 36–42" | 12" × 42" |
| Northeast | 36–48" | 42–48" | 12" × 48" |
| Northern Plains / AK | 48–72" | 54–72" | 12" × 60" |
A standard residential deck post typically requires a 12" diameter footing. For heavier loads (hot tubs, multi-story decks), use 16–18" diameter. The required size depends on soil bearing capacity, tributary load area, and your local building code.
80lb bags are more cost-effective per cubic foot ($0.14/cu ft cheaper on average) and require fewer bags. But 60lb bags are easier to carry and mix, especially in tight spaces or when working alone. For 4+ footings, 80lb bags save significant money.
| Bag Size | Yield (cu ft) | Avg. Price | Cost per Cu Ft | Bags per Cu Yd |
|---|---|---|---|---|
| 50lb bag | 0.375 | $3.80 | $10.13 | 72 |
| 60lb bag | 0.45 | $4.50 | $10.00 | 60 |
| 80lb bag | 0.60 | $5.50 | $9.17 | 45 |
| Ready-mix truck | 27 cu ft | $150–200/yd | $5.56–7.41 | 1 |
Yes, but concrete needs to stay above 50°F for at least 48 hours to cure properly. Below 40°F, use cold-weather concrete mix with accelerators. Never pour on frozen ground — the footing will heave and crack when the soil thaws.
Read our guide
Read our guide
Read our guide
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Last Updated: Mar 13, 2026
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