How Much Concrete for a Round Footing (Sonotube)?
A 12-inch-diameter, 48-inch-deep sonotube holds about 3.14 cubic feet of concrete — that is 6 × 80-lb bags (3.14 ÷ 0.60 = 5.23 → 6). Sonotubes are just round concrete forms, so the math is cylinder volume divided by bag yield. The chart below covers 8-, 10-, and 12-inch tubes.
Key Takeaways
- 12 in × 48 in sonotube = 3.14 cu ft.
- 6 × 80-lb bags (0.60 cu ft yield each).
- Volume = π × radius² × depth.
- Set tubes below the frost line and brace them plumb.
- Add 10% extra for bell-outs at the tube bottom.
How to Calculate Concrete for a Sonotube
A sonotube footing is a perfect cylinder, which makes the volume math clean. Work in feet so the answer comes out in cubic feet.
- 1Measure tube diameter and depth in inches.
- 2Convert to feet: 12 in = 1 ft diameter, 48 in = 4 ft deep.
- 3Radius = diameter ÷ 2 = 0.5 ft.
- 4Volume = π × 0.5² × 4 = 3.14 cu ft.
- 5Bags: 3.14 ÷ 0.60 = 5.23 → 6 eighty-pound bags.
The Sonotube Volume Formula
Dig the hole a few inches wider than the tube so you can backfill and tamp around it for stability.
Volume (cu ft) = π × (diameter ÷ 2)² × depthWorked Example: 12 in × 48 in Sonotube
Full math for a 12-inch tube 48 inches deep, plus a chart for other common tube sizes.
Tube: 12 in diameter × 48 in deep.
In feet: 1 ft diameter (0.5 ft radius) × 4 ft deep.
Volume: 3.14 × 0.5 × 0.5 × 4 = 3.14 cu ft.
80-lb bags: 3.14 ÷ 0.60 = 5.23 → 6 bags.
With 10% extra: 6 × 1.10 = 6.6 → 7 bags.
| Tube size (diameter × depth) | Volume | 80-lb bags |
|---|---|---|
| 8 in × 48 in | 1.40 cu ft | 3 |
| 10 in × 48 in | 2.18 cu ft | 4 |
| 12 in × 36 in | 2.36 cu ft | 4 |
| 12 in × 48 in | 3.14 cu ft | 6 |
Common Mistakes
- Using the diameter as the radius — that quadruples the volume.
- Setting tubes above the frost line, inviting frost heave.
- Pouring without bracing — a full tube is heavy and will lean.
- Cutting the tube flush with grade instead of a few inches above.
- Forgetting a post base bracket and burying the post end in concrete.
Practical Tips
- Wax-coated tubes strip off cleanly after curing; plain tubes can stay buried.
- Drop a few inches of gravel in the hole bottom for drainage.
- Use a post-base bracket with a standoff to keep wood out of standing water.
- Pour in lifts and rod the concrete to remove air pockets.
- Estimate footings plus slab or deck with the NOVORY Concrete Calculator.
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