A 10 inch post hole filled 30 inches deep around a 4x4 post holds 1.15 cubic feet of concrete, about 1.9 bags of 80 lb mix. A 12 inch sonotube filled 4 feet deep holds 3.14 cubic feet of concrete: about 5.2 bags of 80 lb mix, so 6 bags per tube, or 7 bags of 60 lb. Six of them for a deck take 18.85 cubic feet; with a 10% allowance that is 20.73 cubic feet and 35 bags of 80 lb if you mix it all in one go, or 36 if you mix each tube on its own.
How concrete for a sonotube is calculated
A form tube is a cylinder, so its volume is the area of the circle times the height. Work in feet: a 12 inch tube has a radius of 0.5 ft.
Cubic feet = π × (diameter in inches ÷ 24)² × height (ft)
Then divide by the yield of your bag and round up. With a post set in the concrete, the calculator subtracts the post's cross-section over the full depth, since the post fills that space.
For the example: π × 0.5² × 4 = 3.14 cu ft per tube. Six tubes: 18.85 cu ft. Plus 10%: 20.73 cu ft, 0.77 cu yd. At 0.60 cu ft per 80 lb bag (Quikrete's data sheet yield) that is 34.6, so 35 bags.
Bags per tube for 6 to 24 inch tubes
For 4 feet of tube, no allowance, rounded up to whole bags per tube:
| Tube | Concrete | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 6 in | 0.79 cu ft | 2 | 2 |
| 8 in | 1.4 cu ft | 4 | 3 |
| 10 in | 2.18 cu ft | 5 | 4 |
| 12 in | 3.14 cu ft | 7 | 6 |
| 14 in | 4.28 cu ft | 10 | 8 |
| 16 in | 5.59 cu ft | 13 | 10 |
| 18 in | 7.07 cu ft | 16 | 12 |
| 20 in | 8.73 cu ft | 20 | 15 |
| 24 in | 12.57 cu ft | 28 | 21 |
Sonoco, which makes Sonotube forms, publishes the same count for 8, 10, 12 and 16 inch tubes at 4 feet: 3, 4, 6 and 10 bags of 80 lb, and 4, 5, 8 and 13 bags of 60 lb. Its 12 inch figure of 8 bags of 60 lb is one more than the plain math, a margin that the 10% allowance also covers. The calculator shows this table for whatever depth you enter.
Post hole calculator: bags for a fence or mailbox post
Pick the "Post hole" tab above, or use the guide at the bottom of the inputs to size the hole from the post. Quikrete's data sheet says to dig the hole about 3 times the post's width, make it a third of the post's length deep, and place 6 inches of dry mix in the bottom; Sakrete says the same for width and depth and adds 3 to 4 inches of gravel at the base. The calculator subtracts the post from the hole, since the post fills that space. If you put gravel under the concrete, enter it as the gravel base, which is not filled.
Example: ten 10 inch holes 36 inches deep with 6 inches of gravel and a 4x4 post. A 4x4 is 3½ × 3½ inches (Southern Pine Inspection Bureau), so each hole holds (π × 5² − 3.5²) × 30 ÷ 1728 = 1.15 cubic feet of concrete. With 10% extra the ten holes need 12.66 cubic feet: 34 bags of 50 lb Quikrete Fast-Setting Concrete Mix at 0.375 cu ft each, about 3.4 bags a hole. Pick the fast-setting mix in the bag list to see it.
Quikrete post hole sizes by post and length
Hole about 3 times the post width, one third of the post's length deep, concrete to the bottom, no waste, whole bags per hole:
| Post | Post length | Hole | Concrete | 80 lb bags | 60 lb bags | 50 lb fast-setting |
|---|---|---|---|---|---|---|
| 4x4 | 6 ft | 10½ in × 24 in | 1.03 cu ft | 2 | 3 | 3 |
| 4x4 | 8 ft | 10½ in × 32 in | 1.38 cu ft | 3 | 4 | 4 |
| 4x4 | 10 ft | 10½ in × 40 in | 1.72 cu ft | 3 | 4 | 5 |
| 6x6 | 8 ft | 16½ in × 32 in | 3.40 cu ft | 6 | 8 | 10 |
| 6x6 | 10 ft | 16½ in × 40 in | 4.25 cu ft | 8 | 10 | 12 |
A 6x6 is 5½ × 5½ inches, so its hole is 16½ inches wide. Frost depth or a fence's wind load can call for a deeper hole than a third of the post: check your local rule and enter the real depth above.
Sonotubes versus post holes
A sonotube pier is a column: the tube is the form, often poured a little above grade, and a post bracket or anchor bolt sits in the top. Sonoco says its Builder's Tube is for below-grade use and can be poured up to 3 feet above the ground line, with bracing only if it stands 2 feet or more above ground. A post hole is filled around the post itself. The calculator treats both as cylinders and only differs in what it subtracts.
For a deck, the size and depth of each footing is set by the load and your local frost depth. The deck footing calculator sizes them; this page turns the result into bags.
Mixing tube by tube
If you mix and pour each tube separately, round up per tube: 6 bags × 6 tubes = 36, one more than the pooled 35. The calculator shows both.
Next steps
Add a slab or footing to the same order in the concrete calculator, compare bags of concrete in every size, check pads and continuous footings in the concrete footing calculator, or reinforce a footing or slab with the rebar calculator.