80 lb Concrete Bags for Fence Posts
Work out how many bags of concrete each fence post needs.
Your result
Enter your numbers and press the button. Your result appears here.
A 80 lb bag of Quikrete Concrete Mix yields about 0.60 cubic feet.
How the bags are counted
Concrete per hole = (π × (hole width ÷ 2)² − post area) × (depth − gravel). Bags per post = concrete per hole ÷ bag yield, rounded up.
Example: a 4×4 post in a 10.5 in hole 30 in deep, with 6 in of gravel, takes 1.14 ft³ of concrete: 2 bags per post, 20 for 10 posts.
Concrete needed at a glance
Worked out with the same settings as the calculator above, changing only the hole depth.
| Hole depth | Bags per post | Total bags | Concrete |
|---|---|---|---|
| 24 in | 2 | 20 | 8.5 ft³ |
| 30 in | 2 | 20 | 11.4 ft³ |
| 36 in | 3 | 30 | 14.2 ft³ |
| 42 in | 3 | 30 | 17.0 ft³ |
| 48 in | 4 | 40 | 19.9 ft³ |
Bag yields
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb concrete mix | 0.30 ft³ | 90 |
| 50 lb concrete or fast-setting mix | 0.375 ft³ | 72 |
| 60 lb concrete mix | 0.45 ft³ | 60 |
| 80 lb concrete mix | 0.60 ft³ | 45 |
Yields from Quikrete product data. Pour fast-setting mix dry into the hole and add water; it sets in 20 to 40 minutes.
Common questions
How many bags of concrete per fence post?
For a 4×4 post in a hole about 10 to 12 in wide and 2 to 3 ft deep, plan on 2 to 4 bags of 50 lb fast-setting mix. Bigger holes and 6×6 posts take more.
Should I put gravel in the bottom of the hole?
A few inches of gravel under the post helps water drain away from the end grain. Leave the top of the concrete just above the ground and slope it away from the post.
Fast-setting or regular mix?
Fast-setting mix is poured dry into the hole and soaked with water, and it sets in 20 to 40 minutes. Regular mix is cheaper per cubic foot but must be mixed first.
Sources
- Quikrete, Concrete Mix data sheet (bag yields): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
- Quikrete, project calculator (post depth example): https://www.quikrete.com/calculator/