At a glance
- 12 in. sonotube, per foot
- 1.3 bags (80 lb)
- 4×4 post, 10 in. hole, 2 ft deep
- 2 bags (80 lb)
- 80 lb bag yield
- 0.60 cu ft
- Bags in 1 cubic yard
- 45 (80 lb)
Above about one cubic yard of concrete, a ready-mix delivery is usually easier than mixing 45 or more bags.
How to calculate concrete for a sonotube
A sonotube is a round cardboard form, so the concrete inside is a cylinder. To find how much you need:
- Divide the diameter in inches by 24 to get the radius in feet. A 12-inch tube has a 0.5 ft radius.
- Multiply 3.1416 × radius × radius × height in feet to get cubic feet.
- Multiply by the number of tubes, and add about 10% for spills.
- Divide by the bag yield: 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb. Round up.
Concrete per foot of sonotube
| Tube diameter | Cu ft per foot | 80 lb bags per foot | 60 lb bags per foot | 4 ft tube, 80 lb bags* |
|---|---|---|---|---|
| 6 in. | 0.20 | 0.33 | 0.44 | 2 |
| 8 in. | 0.35 | 0.58 | 0.78 | 3 |
| 10 in. | 0.55 | 0.91 | 1.21 | 4 |
| 12 in. | 0.79 | 1.31 | 1.75 | 6 |
| 14 in. | 1.07 | 1.78 | 2.38 | 8 |
| 16 in. | 1.40 | 2.33 | 3.10 | 11 |
| 18 in. | 1.77 | 2.95 | 3.93 | 13 |
| 24 in. | 3.14 | 5.24 | 6.98 | 24 |
*Rounded up to whole bags, including 10% extra.
Tips for pouring sonotubes
- Set the tube on undisturbed soil or a compacted gravel base, and brace it plumb so it can't shift while you fill it.
- Fill in lifts of 1 to 2 feet and tap the sides or rod the concrete to release trapped air.
- Set anchor bolts or post bases while the concrete is still wet.
- Most deck footings must reach below the local frost line, so check your building code for the minimum depth.
How much concrete for a fence post hole?
For a post hole, the concrete fills the space between the hole and the post. The calculator finds the volume of the hole and subtracts the post: a 4×4 (3.5 × 3.5 in.) takes up 0.085 cu ft per foot of depth, and a 6×6 (5.5 × 5.5 in.) takes 0.21 cu ft per foot.
| Post and hole | 2 ft deep | 3 ft deep | 4 ft deep |
|---|---|---|---|
| 4×4 post, 10 in. hole | 2 | 3 | 4 |
| 4×4 post, 12 in. hole | 3 | 4 | 5 |
| 6×6 post, 16 in. hole | 4 | 6 | 8 |
| 6×6 post, 18 in. hole | 6 | 8 | 11 |
| Round steel post (2⅜ in.), 8 in. hole | 2 | 2 | 3 |
Bags of 80 lb concrete per post, rounded up, without extra.
Quikrete recommends a hole 3 times the post diameter, dug to about half the post's above-ground height, and filling with fast-setting mix to 3 to 4 inches below grade, then topping off with soil. If you leave that gap, you'll use slightly less than the full-depth figure here. For a whole fence, our fence cost calculator counts the posts and concrete for you.
Which bag size should you buy?
- 80 lb standard mix (0.60 cu ft): the lowest cost per cubic foot, best for sonotubes and footings.
- 60 lb standard mix (0.45 cu ft): easier to lift and carry; costs a bit more per cubic foot.
- 50 lb fast-setting mix (0.375 cu ft): pour it dry into a post hole and add water, with no mixing. It sets in 20 to 40 minutes, which suits fence and mailbox posts.
Frequently asked questions
How many bags of concrete do I need for a sonotube?
Multiply 3.1416 × radius² × height, all in feet, to get cubic feet, then divide by 0.60 for 80 lb bags or 0.45 for 60 lb bags. A 12-inch sonotube 4 feet tall holds 3.14 cubic feet, which is about 6 bags of 80 lb concrete with 10% extra.
How many 80 lb bags are in a 12-inch sonotube?
A 12-inch sonotube holds 0.785 cubic feet per foot of height, or about 1.3 bags of 80 lb concrete per foot. A 4-foot tube needs about 6 bags with 10% extra, and a full 12-foot tube needs about 16 bags without extra.
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10-inch hole, plan on about 2 bags of 80 lb concrete for a 2-foot hole and 3 bags for a 3-foot hole. A 6×6 post in a 16-inch hole 3 feet deep needs about 6 bags of 80 lb concrete.
How big should a post hole be?
A common rule is a hole about 3 times the width of the post, for example 10 to 12 inches for a 4×4 post, and a depth of about half the post's height above ground. In cold climates, dig below the frost line; check your local building code.
Does the calculator subtract the post?
Yes. In post hole mode, the calculator subtracts the volume of the post from the hole, so you only buy concrete for the space around it. In sonotube mode, the whole tube is filled.
When should I order ready-mix instead of bags?
Above about 1 cubic yard, which is about 45 bags of 80 lb concrete, a ready-mix delivery is usually less work and often cheaper. Compare both in our concrete cost calculator.
How we calculate
Volumes use the cylinder formula π × r² × h. In post hole mode, the post's volume (square for wood posts, using the actual 3.5 in. and 5.5 in. lumber sizes; round for steel) is subtracted from each hole. Bag counts use the manufacturer's stated yields of 0.60 cu ft per 80 lb bag, 0.45 cu ft per 60 lb bag and 0.375 cu ft per 50 lb fast-setting bag, and always round up to whole bags. Cost estimates use $5 to $8 per 80 lb bag and $3.75 to $6 per 60 lb bag, matching our concrete cost calculator; fast-setting bag prices vary too much by brand to show. One cubic yard is 27 cubic feet.
Sources
- Quikrete Fast-Setting Concrete Mix spec data (50 lb yield, post hole sizing)
- Quikrete 80 lb Concrete Mix product details (bag yield)