Footing Formulas
Cubic Yards = (L ft × W ft × D ft) ÷ 27
Round / Cylindrical Post Footing (Sonotube):
Cubic Yards = π × r² × Depth ft ÷ 27
(where r = radius in feet = diameter / 2)
Example — 10" diameter sonotube, 48" deep:
π × (0.417)² × 4 ÷ 27 = 0.081 yd³ per post
12 posts × 0.081 = 0.97 yd³ total
Strip / Wall Footing Reference Table
| Footing Size (W×D) | Linear Ft | Cubic Yards | 80 lb Bags |
|---|---|---|---|
| 12"×8" continuous | 10 ft | 0.31 | 14 bags |
| 12"×8" continuous | 20 ft | 0.62 | 28 bags |
| 16"×8" continuous | 20 ft | 0.82 | 37 bags |
| 16"×12" continuous | 40 ft | 2.47 | 111 bags |
| 24"×12" continuous | 40 ft | 3.70 | 167 bags |
| 24"×12" continuous | 100 ft | 9.26 | ready-mix recommended |
Cylindrical Post Footing Table (Sonotube)
| Tube Diameter | 24" Deep | 36" Deep | 48" Deep |
|---|---|---|---|
| 6" (0.5 ft) | 0.015 yd3 / 1 bag | 0.022 yd3 / 1 bag | 0.029 yd3 / 2 bags |
| 8" (0.67 ft) | 0.026 yd3 / 1 bag | 0.039 yd3 / 2 bags | 0.052 yd3 / 3 bags |
| 10" (0.833 ft) | 0.040 yd3 / 2 bags | 0.060 yd3 / 3 bags | 0.081 yd3 / 4 bags |
| 12" (1 ft) | 0.058 yd3 / 3 bags | 0.087 yd3 / 4 bags | 0.116 yd3 / 6 bags |
| 16" (1.33 ft) | 0.103 yd3 / 5 bags | 0.155 yd3 / 7 bags | 0.207 yd3 / 10 bags |
* Bag count based on 80 lb bags (0.60 cu ft = 0.022 yd3 each). Add 10% extra.
Post footings must extend below the local frost depth to prevent heaving. In northern US: 42–48 inches. In moderate climates: 24–36 inches. Check your local building code — this is often inspected before pouring.
Calculate Concrete for Your Footings
Enter dimensions for strip, pad, or post footings to get exact cubic yards and bag count.
Open Concrete Calculator →FAQ
How do I calculate concrete for footings?
Strip footing: L(ft) × W(ft) × D(ft) ÷ 27 = cubic yards. Round post: π × r²(ft) × depth(ft) ÷ 27. Always add 10% for waste and overpour.
How many bags of concrete for a 10" post footing?
At 36" depth: 0.081 yd3 per post = approximately 3.5 bags of 80 lb concrete. Round up to 4 bags per post.
Do I need rebar in footings?
Yes for most structural footings — check local building codes. Strip footings typically use 2 runs of #4 rebar. Post footings under decks may require a single #4 bar per local code.