Sonotube Calculator
Work out how much concrete fills one or more round form tubes for deck footings, porch posts or columns. Enter the tube diameter in inches, the depth in feet and the number of tubes. Sonotube is a brand name (Sonoco) for cardboard concrete forms; the math is the same for any brand.
How to use the sonotube calculator
- Keep Column / tube selected and enter the tube's inside diameter in inches. Common sizes are 8, 10, 12, 16, 18 and 24 in.
- Enter the height of concrete in the tube in feet: the depth in the ground plus any part that sticks up above grade.
- Set Number of columns to the number of tubes of that size. Run the calculator again for a second size.
- Read the total cubic yards and the bags. The table below shows bags per tube for every common diameter.
Concrete in a round tube
A tube is a cylinder, so its volume is π × radius² × height. With the diameter in inches and the height in feet, the radius in feet is diameter ÷ 24:
Volume (ft³) = π × (diameter ÷ 24)² × height (ft)
A 12 in tube holds π × 0.5² = 0.785 ft³ for every foot of depth. Multiply by the number of tubes, add the waste allowance and divide by 27 for cubic yards. Bags use the QUIKRETE and Sakrete yields: 0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb bag and 0.30 ft³ per 40 lb bag.
Worked example: six 12 in tubes, 4 ft deep
This is the setup loaded above, a common layout for a small freestanding deck.
- One tube: π × 0.5² × 4 = 3.14 ft³.
- Six tubes: 6 × 3.14 = 18.85 ft³, or 0.7 yd³ before waste.
- Add 10%: 20.73 ft³, which is 0.77 yd³.
- Bags: 20.73 ÷ 0.60 = 34.6, rounded up to 35 bags of 80 lb mix (47 of 60 lb, 70 of 40 lb).
That is just under 6 bags per tube, which matches the rule of thumb that a 12 in × 4 ft tube takes about six 80 lb bags. Under 1 yd³, bagged mix and a rented mixer are usually the practical choice.
Choosing the tube size
The tube diameter should come from your deck or porch plans, not from the calculator. The American Wood Council's DCA 6 guide sizes deck footings by how much deck area each post carries and by the soil's bearing capacity: bigger joist spans and weaker soil need wider footings.
A tube usually sits on a wider footing pad or a flared plastic base at the bottom of the hole, which spreads the load. If your plans show one, work out the pad with the footing calculator and add it to the tube volume. The bottom of the footing must be below your local frost line.
Sonotube concrete by diameter: volume per foot and bags per 4 ft tube
| Tube diameter | ft³ per ft of depth | yd³ per ft | 4 ft tube: 80 lb bags | 4 ft tube: 60 lb bags | 4 ft tubes per yd³ |
|---|---|---|---|---|---|
| 6 in | 0.196 | 0.007 | 2 | 2 | 34.4 |
| 8 in | 0.349 | 0.013 | 3 | 4 | 19.3 |
| 10 in | 0.545 | 0.02 | 4 | 6 | 12.4 |
| 12 in | 0.785 | 0.029 | 6 | 8 | 8.6 |
| 14 in | 1.069 | 0.04 | 8 | 11 | 6.3 |
| 16 in | 1.396 | 0.052 | 11 | 14 | 4.8 |
| 18 in | 1.767 | 0.065 | 13 | 18 | 3.8 |
| 20 in | 2.182 | 0.081 | 16 | 22 | 3.1 |
| 24 in | 3.142 | 0.116 | 24 | 31 | 2.1 |
Bag counts include 10% waste. Tubes per yd³ has no waste. Many deck builders flare the bottom of the hole or add a footing pad; add that volume separately.
Tips for pouring tubes
- Measure the real depth of each hole. Holes on a slope are rarely the same depth. Add up the actual heights, or run the calculator for the deepest and shallowest tubes.
- Brace the tube before you fill it. Backfill around the tube and keep it plumb with stakes; a tube that leans or floats changes the post position.
- Fill in lifts and rod the concrete. Pour in layers and work a stick or rebar up and down to remove air pockets, then tap the outside of the tube.
- Set anchors while it's wet. Push post bases or anchor bolts in right after the pour, checked against a string line for height and alignment.
- Buy a spare bag. Holes often end up a little wider than the tube at the bottom. One extra bag is cheaper than a second trip.
Frequently asked questions
How many bags of concrete do I need for a sonotube?
For one 12 in × 4 ft tube, 6 bags of 80 lb mix (3.14 ft³ plus 10% waste). An 8 in × 4 ft tube needs 3 bags, a 10 in tube 4 bags, a 16 in tube 11 bags. See the table for every size.
How much concrete is in a 12 inch sonotube per foot?
About 0.785 ft³ (0.029 yd³) per foot of depth, which is 1.3 bags of 80 lb mix per foot before waste.
How many 4 ft sonotubes can I fill with a yard of concrete?
About 8.6 tubes of 12 in diameter, 12.4 tubes of 10 in, or 4.8 tubes of 16 in, before waste. Divide 27 ft³ by the volume of one tube.
What size sonotube do I need for deck footings?
It depends on the load each post carries and your soil. The AWC DCA 6 guide gives footing sizes by joist span and soil type; your deck plans or building department will specify the diameter and depth.
How deep should a sonotube be?
The bottom of the footing must be below your local frost line and on firm soil. In many northern areas that means 42–48 in or more; in warm climates it can be much shallower. Ask your building department for the local frost depth.
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