Sonotube Concrete Calculator
Find how many bags of concrete fill your round tube forms.
Measure your project
Your answer
That is 3.5 cubic feet, or 0.13 cubic yards. We added 10% extra.
If you buy bags
One 80 lb bag makes 0.6 cubic feet of concrete.
If you order a truck (ready-mix)
Ask for 0.25 cubic yards. We round up to the next quarter yard.
This answer is a guide. Quikrete says tubes under deck posts should go below the frost line. Check your local building rules or ask a builder or engineer.
Which costs less?
But you must mix 6 bags by hand. That is 0.2 tons. For a big job, a truck saves a lot of hard work.
Prices are US middle prices from October 2026. Ask your store for their price.
Quick answers
- A 12 in tube
- 0.8 cubic feet for each foot, or 1⅓ bags of 80 lb, says Sonoco.
- A 12 in × 4 ft tube
- 6 bags of 80 lb, with 10% extra. Quikrete's own table says 6.
- Price of a 12 in tube
- $21.72 for 4 ft, the middle of 4 stores (Oct 2026).
- Sizes sold
- 6–12 in wide for DIY. Quik-Tube is 4 ft long; Sonoco's sheet also lists 8 and 12 ft. Bigger forms go up to 60 in.
How to work it out
- Measure the tube width in inches and its length in feet.
- Halve the width, then divide by 12. That is the radius in feet.
- Multiply 3.142 × radius × radius × length to get cubic feet.
- Add 10% extra. Divide by 0.6 for 80 lb bags, and round up.
Radius: 12 ÷ 2 ÷ 12 = 0.5 ft
3.142 × 0.5 × 0.5 × 4 = 3.14 cubic feet
Add 10%: 3.46 cubic feet
3.46 ÷ 0.6 = 5.8, so 6 bags of 80 lb
Bags per foot: the makers' charts
Sonoco, the maker of Sonotube, prints this chart.
| Tube | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 6 in | 0.2 | ⅓ | ½ |
| 8 in | 0.35 | ⅗ | ⅘ |
| 10 in | 0.55 | 1 | 1¼ |
| 12 in | 0.8 | 1⅓ | 2 |
Quikrete's Quik-Tube sheet counts whole bags instead.
| Tube length | 6 in | 8 in | 10 in | 12 in |
|---|---|---|---|---|
| 1 ft | 1 / 1 | 1 / 1 | 2 / 1 | 2 / 2 |
| 2 ft | 1 / 1 | 2 / 2 | 3 / 2 | 4 / 3 |
| 3 ft | 2 / 1 | 3 / 2 | 4 / 3 | 6 / 4 |
| 4 ft | 2 / 2 | 4 / 3 | 5 / 4 | 7 / 6 |
Each cell: 60 lb bags / 80 lb bags.
Why a 12 in tube gets different answers
Here are the counts we found:
- One store page (Howe Lumber): about 4 bags. It doesn't say what size of bag.
- Sonoco's chart: 1⅓ × 4 = 5⅓ bags of 80 lb.
- Our math: 3.14 cubic feet, or 5.2 bags. Rounded up to whole bags, that is 6.
- Quikrete's table: 6 bags of 80 lb, or 7 of 60 lb.
Sonoco rounds its cubic feet up a little. The store's figure is low, so go by the makers' charts.
Tube sizes and prices
Sonoco's Builder's Tube and Quikrete's Quik-Tube come 6, 8, 10, 12 in wide. Sonoco says these sizes are not exact, so measure if it matters.
Sonoco's sheet lists 4, 8 and 12 ft. Sonoco's 2016 brochure lists 4 ft only. Quik-Tube comes 4 ft long. Bigger Sonotube forms for commercial jobs run from 6 to 60 in wide.
| Tube | Middle price | Range | Stores |
|---|---|---|---|
| 8 in | $12.49 | $11.95–$14.99 | 5 |
| 10 in | $17.60 | $9.99–$19.49 | 3 |
| 12 in | $21.72 | $17.49–$23.49 | 4 |
Sonotube and Quik-Tube prices together. Only 3 stores priced the 10 in tube, so take it as rough. The $9.99 low was one store's 10 in tube. That store's own 8 in cost more.
Bell bases
Sonoco's Tube Base is a bell base: a wider, flared bottom for a tube. It comes in three sizes.
| Base | Fits tubes | Holds | 80 lb bags, no extra |
|---|---|---|---|
| Tube Base TB22 | 8 or 10 in | 2 cubic feet | 4 |
| Tube Base TB28 | 8, 10 or 12 in | 4.5 cubic feet | 8 |
| Tube Base TB32 | 12 to 18 in | 7 cubic feet | 12 |
The calculator doesn't count the base, so add it yourself. A 12 in × 4 ft tube on a TB28 holds 7.6 cubic feet. With 10% extra, that is 15 bags of 80 lb.
Setting the tube
Quikrete's Quik-Tube steps:
- Dig the hole 2 times as wide as the tube.
- Put about 2 in of gravel in the bottom.
- Set the tube so its top ends up 2–6 in above the ground.
For a post, pick a tube at least 2 times as wide as the post. A 4×4 is really 3½ in, so use 8 in or wider.
How deep
No tube maker gives a depth. The IRC, the US home building code, sets deck footings at least 12 in below undisturbed ground. Undisturbed ground is soil no one has dug or filled. If the deck is attached to a house with frost-proof footings, the deck's footings must be frost-proof too. Quikrete says tubes under deck posts go below the frost line.
| Deck area per post | 1,500 psf soil | 2,000 psf soil | 3,000 psf soil |
|---|---|---|---|
| 5 sq ft | 8 × 6 | 8 × 6 | 8 × 6 |
| 20 sq ft | 12 × 6 | 9 × 6 | 8 × 6 |
| 40 sq ft | 16 × 6 | 14 × 6 | 12 × 6 |
| 60 sq ft | 19 × 6 | 17 × 6 | 14 × 6 |
| 80 sq ft | 22 × 7 | 19 × 6 | 16 × 6 |
| 100 sq ft | 25 × 8 | 21 × 6 | 17 × 6 |
| 120 sq ft | 27 × 9 | 23 × 7 | 19 × 6 |
| 140 sq ft | 29 × 10 | 25 × 8 | 21 × 6 |
| 160 sq ft | 31 × 11 | 27 × 9 | 22 × 7 |
The last column is for 3,000 psf soil or firmer. This block is for a 40 psf deck load; the code lists bigger sizes for heavier loads. The thickness is for plain concrete.
The deck area per post is the part of the deck each post holds up. psf means pounds per square foot. For any deck, check your local building rules or ask a builder or engineer.
Questions people ask
How much concrete do I need for a 12 in Sonotube?
About 0.8 cubic feet for each foot of tube, says Sonoco. That is 1⅓ bags of 80 lb per foot.
How much concrete to fill a Sonotube?
Sonoco's chart, in cubic feet per foot of tube: 6 in takes 0.2. 8 in takes 0.35. 10 in takes 0.55. 12 in takes 0.8. Multiply by the length in feet.
How many bags for a 12 in × 4 ft Sonotube?
Our math gives 5.2 bags of 80 lb, so buy 6. Quikrete's table also says 6. Sonoco's chart gives 5⅓.
How deep should a Sonotube go?
No tube maker gives a number. The IRC puts deck footings at least 12 in below undisturbed ground. Quikrete says deck posts go below the frost line.
How much does a Sonotube cost?
About $12.49 for an 8 in × 4 ft tube, the middle of 5 stores. A 10 in is about $17.60, from only 3 stores, so it is rough. A 12 in is about $21.72, from 4 stores.
What size Sonotube for a deck post?
At least twice the post width, says Quikrete. For a 4×4 (really 3½ in), that is 8 in or wider. The footing must also meet the IRC deck footing table (R507.3.1).
Where our numbers come from
- Tube sizes and concrete per foot: Sonoco Sonotube Builder's Tube sheet
- Bigger tubes and bell bases: Sonoco Sonotube brochure
- Bags to fill a tube: Quikrete Quik-Tube data sheet
- Tube footings for posts: Quikrete Quik-Tube pillar project
- 12 in tube store page: Howe Lumber (says about 4 bags)
- Bag sizes and weight: Quikrete Concrete Mix data sheet
- Extra for spills: Sakrete slab calculator (10%)
- Deck footings: US home building code (IRC R507.3)
- Real size of a 4×4 and 6×6: NIST lumber standard PS 20
- Tube form prices: 3 to 5 US stores per size (Oct 2026)
- Bag price: the middle price of 9 US stores (Oct 2026)
- Truck price: the middle price of 5 US concrete sellers (Oct 2026)
- Small-order fee: the middle of 4 US cost guides (2026)
Made by BuildersCount. Last checked 3 Oct 2026.
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