Sonotube Calculator
Calculate concrete needed for Sonotube and cylindrical pier forms — in cubic yards and bags.
Your Sonotube Estimate
- Enter tube diameter — as the "Width" field. Common sizes: 8", 10", 12", 14", 16", 18", 24".
- Enter the depth — use the thickness field for tube depth. Presets cover common frost line depths.
- Set "Length" to 1 — for a single tube. For multiple identical tubes, enter the number of tubes as Length and 1 as Width.
- Add waste — 10% covers spillage and overfill. The top of the tube usually gets a slight crown.
- Review results — per-tube volume is shown so you can batch your mix correctly.
Concrete per Sonotube by Size and Depth
| Tube Diameter | Depth 36" | Depth 48" | Depth 60" | 80 lb Bags (48" deep) |
|---|---|---|---|---|
| 8" | 0.058 yd³ | 0.078 yd³ | 0.097 yd³ | 4 |
| 10" | 0.091 yd³ | 0.121 yd³ | 0.152 yd³ | 7 |
| 12" | 0.131 yd³ | 0.175 yd³ | 0.218 yd³ | 9 |
| 14" | 0.178 yd³ | 0.238 yd³ | 0.297 yd³ | 13 |
| 16" | 0.233 yd³ | 0.310 yd³ | 0.388 yd³ | 17 |
| 18" | 0.295 yd³ | 0.393 yd³ | 0.491 yd³ | 21 |
| 24" | 0.524 yd³ | 0.698 yd³ | 0.873 yd³ | 37 |
Bag counts include 10% waste. Values are per single tube.
Setting Sonotubes Correctly
Sonotubes (cardboard concrete form tubes) should extend at least 2 inches above grade to prevent ground contact with the post hardware. The bottom of the tube must sit below the frost line on undisturbed soil or a compacted gravel pad. Flaring the bottom of the hole wider than the tube creates a bell footing that resists uplift from frost heave.
Before pouring, brace each tube plumb using diagonal stakes. Wet the inside of the tube and the surrounding soil lightly — dry cardboard and soil will absorb water from the concrete and weaken the mix at the edges. Pour in lifts of 12 to 18 inches, rodding (poking with a stick) each lift to remove air pockets.
Formulas Used in This Calculator
Sonotubes are cylinders, so the calculator uses the cylinder volume formula:
Multiple tubes = Single-tube volume × Number of tubes
Cubic yards = Total volume (ft³) ÷ 27
Bags (80 lb) = Total volume (ft³) ÷ 0.6
With waste = Results × (1 + Waste %)
Enter the diameter in inches in the Width field and depth in inches in the Thickness field — the calculator converts to feet internally.
Related Calculators
Sonotubes are the fastest way to form round concrete footings. See the full construction calculator collection.
Frequently Asked Questions
How many bags of concrete for a 12-inch Sonotube?
A 12-inch diameter Sonotube at 48 inches deep needs approximately 0.175 cubic yards — that's about 9 bags of 80 lb mix (including 10% waste). At 36 inches deep, it drops to about 7 bags.
How deep should a Sonotube footing be?
Sonotube footings for deck posts should extend below the local frost depth — typically 36–48 inches in northern climates, 12–18 inches in the south. Check your local building code for the required frost depth. The bottom should sit on undisturbed soil.
Can I use quick-setting concrete in Sonotubes?
Yes, and it is common. Products like Quikrete Fast-Setting Concrete are poured dry into the tube with the post set, then water is added. This sets in 20–40 minutes and allows immediate continuation of work. Follow the manufacturer water ratio to ensure full strength.
How do I choose the right Sonotube diameter?
Sonotube diameter is typically set by the post size plus bearing area needed. A common rule: use a tube diameter 2–3 times the post width. For a 4×4 post (3.5 inches), a 10-inch tube is typical. For a 6×6 post, use a 12-inch tube. For deck footings in normal soil (2,000 psf bearing), a 12-inch diameter footing at 48 inches deep supports approximately 3,500 lbs — enough for most deck corner posts. Check local code for your specific footing schedule once you have chosen a diameter and depth. Confirm sizing requirements for your structure and soil conditions.
Should I bell out the bottom of a Sonotube?
A belled footing can increase bearing area and help resist uplift in some designs. Use this calculator to plan volume, then confirm whether a bell shape is needed for your project and local conditions.