How to calculate concrete for a Sonotube
A round form is a cylinder, so the volume is π × radius² × length. With the diameter in inches and the length in feet: cu ft = π × (diameter ÷ 24)² × length. Divide by the bag yield — 0.60 cu ft for an 80-lb bag, 0.45 for a 60-lb bag — and round up.
Worked example — a 12 in tube, 4 ft long: π × 0.5² × 4 = 3.14 cu ft. That is 5.24 × 80-lb bags (buy 6) or 6.98 × 60-lb bags (buy 7). For several tubes, multiply first and round once.
Concrete per foot of tube
| Diameter | Cu ft per ft | 80-lb bags per 4 ft | 60-lb bags per 4 ft |
|---|---|---|---|
| 6 in | 0.196 | 2 | 2 |
| 8 in | 0.349 | 3 | 4 |
| 10 in | 0.545 | 4 | 5 |
| 12 in | 0.785 | 6 | 7 |
| 14 in | 1.069 | 8 | 10 |
| 16 in | 1.396 | 10 | 13 |
| 18 in | 1.767 | 12 | 16 |
| 24 in | 3.142 | 21 | 28 |
What the tube makers say
Quikrete's Quik-Tube data sheet and Sonoco's Sonotube Builder's Tube page print bag counts for 4 ft of tube. They agree with the geometry except one cell: Sonoco rounds a 12 in tube up to 8 × 60-lb bags where the math (and Quik-Tube) says 7.
| Tube, 4 ft | Quik-Tube (60 / 80 lb) | Sonoco (60 / 80 lb) |
|---|---|---|
| 6 in | 2 / 2 | — |
| 8 in | 4 / 3 | 4 / 3 |
| 10 in | 5 / 4 | 5 / 4 |
| 12 in | 7 / 6 | 8 / 6 |
| 16 in | — | 13 / 10 |
Setting the tube
- Dig a hole about 2× the tube diameter, set the tube and backfill around it so it stays put (Quik-Tube instructions).
- For a footing under the tube: about 2 in of gravel, one vertical rebar driven into the center, then a 4–6 in footing. Set the tube on the footing and fill it. The calculator adds the footing concrete when you switch it on.
- Builder's tubes are meant for use below grade and up to 3 ft above it. Brace them with a collar or frame once 2 ft or more sticks up.
- Rod the concrete as you fill to remove air pockets (Quikrete says not to use a mechanical vibrator in Quik-Tubes), then set post bases or anchor bolts in the wet top.
- The bottom must sit at least 12 in below undisturbed ground (IRC R507.3.2 for decks) and below your frost depth.
How many tubes to buy
Tubes are sold in 4 ft lengths at home centers, with 10 and 12 ft lengths in some regions. Cut short piers from one tube: two 2 ft piers come out of a single 4 ft tube, so the calculator rounds the tube count for the whole job, not per pier. Home Depot listed Sakrete 48 in tubes at $10.88 for 8 in and $14.48 for 10 in in July 2026; larger diameters cost more. A pier longer than 4 ft should come from one long tube rather than two short ones stacked.
Bags or a truck for a row of piers?
A single tube is a bag job. A deck's worth adds up fast: nine 12 in tubes 4 ft long hold 28.3 cu ft, or 52 × 80-lb bags with 10% extra — 4,160 lb to lift and pour through a tube opening. Ready-mix is sold by the quarter yard with a 1 yd minimum, so the same job is a 1.25 yd³ order at about $528 with fees, against $308 in bags. Ask whether the truck's chute can reach every hole; a wheelbarrow and a bucket work for the rest. Mixing your own? 60-lb bags are easier to lift to chest height over a tall tube, at about one more bag per 4 ft of 12 in tube.
Common mistakes
- Ordering by the tube length instead of the concrete length — only the filled part counts.
- Rounding each tube up and buying a pile of extra bags: for many tubes, round the total.
- Forgetting the footing under the tube; a wide footing often holds more concrete than the tube itself.
- Letting a tall tube drift out of plumb while filling — brace it first.
Square pads instead of tubes? Use the footing calculator. Setting posts directly in concrete? See the post hole calculator.