How much concrete a form tube takes
A round column is π/4 times its diameter squared times its height. A 10 in form tube 4 ft tall is π/4 × (10/12)² × 4 = 2.18 cubic feet, which is 3.64 bags of 80 lb concrete mix before waste. The four this page opens with — the footings under a small deck or a porch post each — take 8.73 ft³, 9.60 with the 10% waste in the box, so 16 bags of 80 lb, 1,280 lb, and about 12 gallons of water to mix them. Buy for all the columns at once and round up once: four tubes rounded up one by one would buy 16 bags as well here, but at other sizes the difference is a bag a tube.
The diameter is typed in inches and the height in feet, the way tubes are sold and holes are dug. Change the quantity to the number of columns. The shopping list counts the tubes themselves too, one line per diameter and height, so the list you take to the store has them on it.
Concrete per foot of tube
Before waste, per foot of height:
| Tube | Cubic feet per foot | 80 lb bags per foot | 60 lb bags per foot |
|---|---|---|---|
| 8" | 0.349 | 0.58 | 0.78 |
| 10" | 0.545 | 0.91 | 1.21 |
| 12" | 0.785 | 1.31 | 1.75 |
| 14" | 1.069 | 1.78 | 2.38 |
| 16" | 1.396 | 2.33 | 3.10 |
| 18" | 1.767 | 2.95 | 3.93 |
| 24" | 3.142 | 5.24 | 6.98 |
Multiply by the height of each tube and the number of tubes, add the waste, and round up once. Doubling the diameter quadruples the concrete: a 24 in tube takes nine times what an 8 in one does.
How tall to type the tube
Type the concrete's height, from the bottom of the hole to the top of the tube: the part below ground and the few inches most tubes stand above it. The bottom of a deck or porch footing sits below the frost line, a depth set by your building department, and that is what the panel under the answer says it cannot check. A footing that sits above the frost line is lifted a little every winter, and no amount of concrete in the tube fixes it.
A wider base
Many column footings spread out at the bottom, with a flared plastic base or a square pad under the tube, to bear on more soil. This page does not draw the flared kind yet. A square pad can be counted as a second part: switch to the concrete footing calculator, which takes a column and a footing in the same project, and type the pad as a short footing — its length, its width and its thickness.
Common questions
How many bags of concrete for a 12 inch tube?
0.785 ft³ for every foot of height: 3.14 ft³ for a 4 ft tube, which is 5.24 bags of 80 lb or 6.98 of 60 lb before waste. Round the whole job up, not each tube.
How many bags for an 8 inch tube 4 ft deep?
1.40 ft³, which is 2.33 bags of 80 lb before waste — so 3 bags for one tube, and fewer per tube when you buy for several at once.
Is a form tube the same as a Sonotube?
Sonotube is a brand name for a cardboard concrete form tube. This page counts the concrete for any round form of the same size, whoever makes it, and we are not connected with Sonoco or any other maker.
Does this count the rebar in the column?
No. The steel in a column is sized with the column, by whoever designed it, and comes from a drawing. The concrete, the bags, the water and the tubes are here.
Is the waste added twice?
No. Waste is multiplied in exactly one place in this site, and the result shows the volume before and after it. Set the box to 0 and the two lines agree.