Fence Post Concrete Calculator
Enter the hole diameter, the depth and the number of posts. The count is for the full hole, so it runs slightly high, which covers waste.
How much concrete per fence post?
The table subtracts the space the post takes up, so it is the concrete you actually pour around one post.
| Post | Hole diameter | Hole depth | Cubic feet | 50 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| 4 × 4 post (3.5 in actual) | 12 in | 24 in | 1.40 | 4 | 3 |
| 4 × 4 post (3.5 in actual) | 12 in | 30 in | 1.75 | 5 | 3 |
| 4 × 4 post (3.5 in actual) | 12 in | 36 in | 2.10 | 6 | 4 |
| 6 × 6 post (5.5 in actual) | 18 in | 24 in | 3.11 | 9 | 6 |
| 6 × 6 post (5.5 in actual) | 18 in | 30 in | 3.89 | 11 | 7 |
| 6 × 6 post (5.5 in actual) | 18 in | 36 in | 4.67 | 13 | 8 |
For a whole fence, multiply by the number of posts. A 100 ft fence with posts every 8 ft has about 14 posts.
Sizing the hole
- Diameter: about three times the post width. The QUIKRETE calculator uses a 12-inch hole for a 4-inch post and an 18-inch hole for a 6-inch post.
- Depth: one-third to one-half of the post height above ground is a common rule. A 6 ft fence usually means a hole 24 to 36 inches deep.
- Frost: where the ground freezes, the hole should reach below the frost line. Your building department can tell you the depth.
The formula
Hole volume = 3.1416 × (diameter ÷ 2)² × depth. Subtract the post: post width × post width × depth. Keep everything in feet.
Example for a 4 × 4 post, 12-inch hole, 30 inches deep: the hole is 3.1416 × 0.5 × 0.5 × 2.5 = 1.96 cubic feet. The post takes 0.292 × 0.292 × 2.5 = 0.21. That leaves 1.75 cubic feet.
Which concrete to use
Fast-setting mix in 50 lb bags is the usual choice for posts because it sets quickly without bracing for long. Standard concrete mix also works and is stronger for gate posts and corner posts that carry more load.
Frequently asked questions
How much concrete do I need per fence post?
For a 4 × 4 post in a 12-inch hole 30 inches deep, about 1.75 cubic feet. That is 5 bags of 50 lb fast-setting mix or 3 bags of 80 lb mix.
How many bags of concrete per fence post?
Usually 4 to 6 bags of 50 lb mix for a 4 × 4 post, depending on hole depth. A 6 × 6 post in an 18-inch hole needs far more, about 9 to 13 bags.
How deep should a fence post hole be?
A common rule is one-third to one-half of the post height above ground, and below the frost line where the ground freezes. Local codes can set a minimum.
How wide should the hole be?
About three times the width of the post: a 12-inch hole for a 4 × 4 post.
Do I need to mix fast-setting concrete?
Fast-setting mixes are made to be poured dry into the hole and then soaked with water. Follow the directions on the bag you buy.