How Many Bags of Concrete for 40 Fence Posts?
Setting 40 posts takes more concrete than most people expect. With a typical 10-inch hole, 30 inches deep, around a 4x4 post, how many bags of concrete for 40 fence posts comes to about 77 bags of 80 lb mix, or 103 bags of 60 lb.
Concrete calculator →Bags by post and hole size for 40 posts
| Post | Hole | Depth | Cu ft per hole | 80 lb bags (all) | 60 lb bags (all) |
|---|---|---|---|---|---|
| 4x4 | 10 in | 24 in | 0.92 | 62 | 82 |
| 4x4 | 10 in | 30 in | 1.15 | 77 | 103 |
| 4x4 | 12 in | 36 in | 2.10 | 141 | 187 |
| 6x6 | 14 in | 36 in | 2.58 | 172 | 230 |
| 6x6 | 16 in | 42 in | 4.15 | 277 | 370 |
Volumes subtract the post (3.5 or 5.5 inches square) from a round hole and assume 0.60 cu ft per 80 lb bag and 0.45 per 60 lb bag.
Related: How Many Bags of Concrete for 4 Fence Posts?
Hole size rules of thumb
Many installers dig holes around three times as wide as the post and bury about one-third of the post. In cold climates, going below frost depth keeps posts from heaving. Corners, ends and gate posts take the most load and deserve the largest holes.
Gravel at the bottom
Put 3 to 6 inches of gravel under each post for drainage and tamp it. Crown the concrete at the top so rain runs off instead of pooling against the wood.
Related: How Many Bags of Concrete for 6 Fence Posts?
Fast-setting mix or regular
For many posts, regular concrete mixed in batches is economical; fast-setting mix saves time when you want to keep moving down the line. Either way, brace each post plumb until the concrete sets. See fast-setting concrete for posts.
Buy a few extra
Holes are rarely dug to the exact size. Add about 10 percent to the bag count, or 85 bags for 40 posts in 10-inch holes, and return what you do not open.
Questions people ask
How many 80 lb bags for 40 fence posts?
About 77 bags for 4x4 posts in 10-inch holes, 30 inches deep.
How many 60 lb bags for 40 fence posts?
About 103 bags for the same holes.
How big should a fence post hole be?
About three times the post width, deep enough for roughly a third of the post length or below frost depth.
How many bags for 40 6x6 posts?
About 172 bags of 80 lb mix in 14-inch holes, 36 inches deep.
Should I put gravel under fence posts?
Yes, a few inches of gravel at the bottom helps drainage.
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