Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Height (depth of fill) 4 ft
- Tube diameter = 1 ft (12 in)
- Volume = π × 0.5² × 4 × 1 = 3.1416 ft³
- Net volume = 3.1416 ft³ ÷ 27 = 0.1164 yd³ (× 0.0283168 = 0.089 m³)
- Order quantity = 0.1164 yd³ × (1 + 10%) = 0.128 yd³ = 3.4558 ft³
- 80 lb bags = 3.4558 ft³ ÷ 0.6 ft³ per bag = 5.76 → 6 bags (always round up)
- Weight = 3.4558 ft³ × 150 lb/ft³ = 518 lb
How to measure
Sonotube is the best-known brand of round fiber forms, and people use the name for any cardboard tube form. Use the tube size on the label, which is the inside diameter in inches. If your form is a different size, choose “Custom” and type the diameter.
Height is how far you fill the tube, not the tube length. A tube cut to 4 ft that is buried 1 ft deep and filled to the top still holds concrete for 4 ft of height. If the pier will sit on a footing pad, calculate the pad on its own.
Concrete per foot for each tube size
The volume of a tube depends on the square of its diameter. Doubling the diameter quadruples the concrete per foot, which is why a 24 in tube is not “twice” a 12 in tube; it is four times. Use the table to size a pour in your head.
| Tube | ft³ per ft | 80 lb bags per ft | Feet per yd³ |
|---|---|---|---|
| 6 in | 0.196 | 0.33 | 137.5 |
| 8 in | 0.349 | 0.58 | 77.3 |
| 10 in | 0.545 | 0.91 | 49.5 |
| 12 in | 0.785 | 1.31 | 34.4 |
| 14 in | 1.069 | 1.78 | 25.3 |
| 16 in | 1.396 | 2.33 | 19.3 |
| 18 in | 1.767 | 2.95 | 15.3 |
| 20 in | 2.182 | 3.64 | 12.4 |
| 24 in | 3.142 | 5.24 | 8.6 |
| 30 in | 4.909 | 8.18 | 5.5 |
| 36 in | 7.069 | 11.78 | 3.8 |
Worked example: six 12 in piers, 4 ft high
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Height (depth of fill) 4 ft
- Tube diameter = 1 ft (12 in)
- Volume = π × 0.5² × 4 × 6 = 18.8496 ft³
- Net volume = 18.8496 ft³ ÷ 27 = 0.6981 yd³ (× 0.0283168 = 0.5338 m³)
- Order quantity = 0.6981 yd³ × (1 + 10%) = 0.7679 yd³ = 20.7345 ft³
- 80 lb bags = 20.7345 ft³ ÷ 0.6 ft³ per bag = 34.56 → 35 bags (always round up)
- Weight = 20.7345 ft³ × 150 lb/ft³ = 3,110 lb
| Quantity | Result |
|---|---|
| Net volume | 0.698 yd³ (18.85 ft³, 0.534 m³) |
| Order with 10% waste | 0.77 yd³ (0.59 m³) |
| 80 lb bags | 35 |
| 60 lb bags | 47 |
| Weight at 150 lb/ft³ | 3,110 lb |
Six piers need 0.77 yd³, or 35 80 lb bags, which is about 6 bags per tube. That is a bagged job for a weekend. A truck order this small usually triggers a short-load fee.
How deep should the tubes go?
The bottom of a footing tube must sit below the frost line in cold climates, and on firm, undisturbed soil. Frost depth ranges from nothing in warm regions to over 4 ft in cold ones, so look it up with your building department. Many deck codes also require a spread footing under the tube, in which case you pour the pad first and set the tube on top.
Tube depth and diameter come from the structural plans. This calculator tells you how much concrete they hold; it does not size them.
Setting tubes so the concrete behaves
- Set the tube plumb and brace it so it cannot move while you fill.
- Fill in lifts and rod or vibrate the concrete to remove air pockets.
- Place anchor bolts or post bases while the concrete is wet, using a template for exact position.
- Strip the paper only after the concrete has gained strength. Cure under plastic or with a damp cover in hot weather (see the curing guide).
- Keep the top surface a little above grade and slope it away so water does not pool at the post base.
Mistakes to avoid
- Using the outside diameter of the tube. The label is the inside diameter.
- Forgetting the footing pad below the tube.
- Ordering by the number of tubes without checking the height of each. Piers on a slope have different heights.
- Ignoring waste. A cardboard form bulges as the concrete goes in, and spillage is routine.
Frequently asked questions
How much concrete is in a 12 in Sonotube?
A 12 in tube holds 0.785 ft³ per foot of height. A 4 ft fill is 3.14 ft³, or 0.116 yd³.
How many bags does a 12 in tube take?
About 1.3 bags of 80 lb mix per foot of height. A 4 ft fill takes 5.2 bags with no waste, so buy 6.
How many 8 in tubes can I fill from one yard of concrete?
An 8 in tube holds 0.349 ft³ per foot, so one cubic yard fills 77 ft of tube. That is 19 tubes of 4 ft each before waste.
Is Sonotube a brand name?
Yes. Sonotube is a trademark of Sonoco. The calculator works for any round fiber or plastic form.
Do I need rebar in a tube?
Often yes, especially for taller piers or where the plans call for it. Rebar takes up very little volume, so you do not subtract it from the concrete.
What size tube for a deck footing?
The size comes from the deck design and the soil. Common deck piers range from 8 to 16 in. Check your plans or local requirements.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.