Guides · Concrete
Fence Post Concrete: How Many Bags per Post?
By the MyBuildCalcHub team · Figures verified against our audited estimating engine · Updated
The per-post math, honestly done
A post hole is a cylinder with a post-shaped hole in the middle, and skipping the subtraction over-orders by about 15–20%. The concrete volume is π × (hole radius)² × depth − post cross-section × depth. For the classic setup, a 4×4 post (actual size 3.5 × 3.5 in) in a 10-inch hole, 24 inches deep, that's 1.09 ft³ of cylinder minus 0.17 ft³ of post = 0.92 cubic feet of concrete, which is two 80 lb bags per post with a little left in the second bag for crowning the top.
Bags per post, by post and hole size
| Post (actual) | Hole Ø | Depth | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4×4 (3.5 in) | 10 in | 24 in | 0.92 ft³ | 2 | 3 |
| 4×4 (3.5 in) | 10 in | 30 in | 1.15 ft³ | 2 | 3 |
| 4×4 (3.5 in) | 12 in | 24 in | 1.40 ft³ | 3 | 4 |
| 6×6 (5.5 in) | 12 in | 30 in | 1.44 ft³ | 3 | 4 |
| 6×6 (5.5 in) | 14 in | 30 in | 2.15 ft³ | 4 | 5 |
Counts are per post, rounded up, and rounding up per post (rather than across the whole fence) is deliberate, because posts are usually set one or two at a time and a partial bag doesn't keep once mixed.
Whole-fence worked example
A 12-post privacy fence on 4×4s, holes 10 × 24 in: 12 × 0.92 = 11.0 cubic feet, call it 12.1 ft³ with the standard 10% buffer, about 21 × 80 lb bags, a comfortably manageable pallet-corner rather than a truck order. That's the general pattern for fences: lots of small, separated pours is exactly the scenario where bags beat ready-mix, since you're setting posts over hours or days and a truck's clock never stops.
Setting rules that outlast the fence
Dig to 3× the post width and to a third of the above-ground height (a 6 ft fence wants 24–36 in holes), always below the frost line your building department quotes, posts that heave every winter were almost always set above it. Add 3–4 inches of gravel under the post for drainage, since end grain standing in a concrete cup of water is how posts rot from the bottom. Slope or crown the concrete's top surface away from the wood, keep posts braced plumb while the mix sets, and pour each hole in one go, even at this small scale, a two-batch hole is a weaker hole.
Fast-setting vs. standard mix for posts
Post setting is the one concrete job with two legitimate products. Standard mix is mixed wet, placed, and braced for a day, strongest, cheapest per bag, and right for gates and structural corners. Fast-setting post mix is designed to be poured dry into the hole around the braced post, watered from the top per the bag's directions, and hard enough to release braces in 20–40 minutes, which is why fence crews love it: a two-person team can set an entire line in a day without a mixer on site. Yields per bag are comparable between the two, so every count in the table above holds for either product. One discipline applies to both: pour each hole in one continuous fill, since even a post-sized pour can form a weak seam between lifts.
Frequently asked questions
How many bags of concrete do I need per fence post?
A standard 4x4 post set in a 10-inch hole, 24 inches deep, takes about 2 x 80 lb bags (0.92 cubic feet of concrete after subtracting the post). Widen the hole to 12 inches and it becomes 3 bags; a 6x6 post in a 12-inch, 30-inch hole also takes 3.
How deep should a fence post hole be?
A fence post hole should be one-third to one-half of the post's above-ground height, with 24 inches as the practical minimum, and always below your local frost line, which your building department can confirm. A 6-foot privacy fence therefore wants holes 24-36 inches deep.
How wide should the hole be around a fence post?
The hole diameter should be about three times the post width: roughly 10-12 inches for a 4x4 post and 14-16 inches for a 6x6. The concrete collar around the post is what resists wind leverage, so a too-narrow hole undermines the whole fence.