How Many Bags of Concrete Per Fence Post?
A fence post’s concrete needs depend on three things: how thick the post is, how wide the hole is, and how deep it goes. Get those three right and the bag count follows a simple, repeatable formula — the same one whether you’re setting three posts or thirty.
Quick answer
A standard 4×4 post in a 10-inch hole, 2 feet deep, needs about 2 bags of 80 lb concrete (or 3 bags of 50 lb). A larger 6×6 gate or corner post in a wider 16-inch hole needs closer to 6 bags. Multiply your per-post number by your total post count for the whole fence line.
For your exact post size, hole dimensions, and total post count, the Fence Post Concrete Calculator gives you an instant bag count, with the post’s own volume already subtracted.
Hole size by post size
| Post size | Actual dimensions | Recommended hole diameter |
|---|---|---|
| 4×4 | 3.5 × 3.5 in | 10-12 in |
| 6×6 | 5.5 × 5.5 in | 16-18 in |
| Round post (4 in) | 4 in diameter | 10-12 in |
A hole that’s too narrow doesn’t leave enough concrete around the post to resist wind and lateral force. Too wide wastes material without adding real stability — the 3x rule is the sweet spot most fence installers actually use.
How deep the hole should go
| Fence height | Recommended hole depth |
|---|---|
| 4 ft | 18-24 in |
| 6 ft | 24-30 in |
| 8 ft | 30-36 in |
Check your local frost depth too — in cold climates, the hole may need to go below the frost line regardless of fence height, or the post can shift as the ground freezes and thaws.
Why getting this right matters
Undersized holes are the most common reason a fence leans or shifts within a few years — the post simply doesn’t have enough concrete anchoring it against wind load. Oversized holes aren’t dangerous, just wasteful, since you’re buying and mixing more bags than the post actually needs. Gate posts and corner posts carry more stress than a straight run of fence line, which is exactly why they typically get a wider hole and more concrete than the posts in between.
The formula
Radius is half the hole’s diameter, in feet. Post volume is the post’s actual cross-section (3.5×3.5 in for a “4×4”) multiplied by how deep it’s actually buried.
What to measure first
- Post sizeUse the actual dimension (3.5 in for a 4×4), not the nominal name.
- Hole diameterRoughly 3 times the post’s width.
- Hole depthA third to half the post’s above-ground height, or deeper if your local frost line requires it.
- Total post countCount corner and gate posts separately if they’ll use a wider hole than the rest of the line.
Worked examples
Example 1 — 4×4 post, 10 in hole, 2 ft deep, 80 lb bags
- Hole volume3.14 × 0.417² × 2 = 1.09 cu ft
- Post volume0.292 × 0.292 × 2 = 0.17 cu ft
- Net volume, +10% waste1.01 cu ft
1.01 ÷ 0.60 = 1.69, rounded up to 2 bags.
Example 2 — 6×6 gate post, 16 in hole, 2.5 ft deep, 80 lb bags
- Hole volume3.14 × 0.667² × 2.5 = 3.49 cu ft
- Post volume0.458 × 0.458 × 2.5 = 0.53 cu ft
- Net volume, +10% waste3.26 cu ft
3.26 ÷ 0.60 = 5.43, rounded up to 6 bags — a gate post genuinely needs three times the concrete of a standard line post.
Example 3 — a full fence line, 10 posts, 4×4, 10 in holes
Using the 2-bag figure from Example 1: 10 posts × 2 bags = 20 bags of 80 lb concrete for the whole run. Add extra separately for any wider gate or corner post holes.
Get your exact fence total.
Enter your post size, hole dimensions, and post count for an instant bag count for the whole fence line.
Use the free Fence Post Concrete CalculatorDry-pour vs. wet-mix: which method to use
| Method | How it works | Best for |
|---|---|---|
| Dry-pour | Pour dry mix straight into the hole around the post, then water it in place | Fast-setting mix, single posts, minimal tools |
| Wet-mix | Mix concrete in a wheelbarrow or tub first, then pour it in | Standard mix, larger posts, more consistent strength |
Dry-pour is faster and needs no mixing equipment, which is why it’s the go-to for a handful of fence posts. Wet-mix generally gives more consistent results on larger posts like 6×6 gate posts, where getting the water distributed evenly through a dry-poured mix is harder to control.
Quick reference: bags per post
| Post & hole | Depth | 80 lb bags | 50 lb bags |
|---|---|---|---|
| 4×4, 8 in hole | 1.5 ft | 1 | 2 |
| 4×4, 10 in hole | 2 ft | 2 | 3 |
| 4×4, 12 in hole (gate/corner) | 2.5 ft | 4 | 6 |
| 6×6, 12 in hole | 2.5 ft | 3 | 5 |
| 6×6, 16 in hole (gate/corner) | 2.5 ft | 6 | 9 |
Mistakes that throw off a fence post estimate
- Using nominal post size instead of actualA “4×4” is really 3.5×3.5 inches — using 4×4 exactly slightly overstates the post’s volume.
- Digging every hole the same sizeGate and corner posts carry more load and genuinely need a wider hole than line posts.
- Not checking local frost depthA hole too shallow for your climate can let the post heave as the ground freezes.
- Forgetting to subtract the post’s own volumeOver-orders material on every single hole in the project.
- Rounding a small bag count down1.69 bags means 2, not 1.
- Skipping the gravel baseA few inches of gravel at the bottom of the hole improves drainage and helps prevent the post from sitting in standing water.
What can change the number
- Post sizeA 6×6 post displaces meaningfully more volume than a 4×4 at the same hole size.
- Gate and corner postsTypically need a wider hole and more concrete than straight-line posts.
- Soil typeSandy or loose soil often benefits from a slightly deeper hole for extra stability.
- Local frost depthColder climates require deeper holes regardless of fence height.
Calculate it in five steps
- Pick your post size and use its actual, not nominal, dimensions.
- Size the hole at roughly 3 times the post’s width.
- Set the depth at a third to half the post’s above-ground height, or your local frost depth.
- Subtract the post’s volume from the hole volume, add 10% waste.
- Divide by bag yield, round up, and multiply by your post count.
Frequently asked questions
How many bags of concrete for one 4×4 fence post?
About 2 bags of 80 lb concrete for a standard 10-inch hole, 2 feet deep, once the post’s own volume is subtracted.
How many bags for a 6×6 gate post?
Around 6 bags of 80 lb concrete for a 16-inch hole, 2.5 feet deep — gate posts need a wider hole than standard line posts.
How wide should a fence post hole be?
Roughly 3 times the post’s width — about 10-12 inches for a 4×4, and 16-18 inches for a 6×6.
How deep should a fence post hole be?
A third to half of the post’s above-ground height, though colder climates may require going deeper to reach below the local frost line.
Do I need to subtract the post’s volume from my calculation?
Yes, for an accurate count — the post itself displaces real volume inside the hole. Skipping this step means slightly over-ordering material.
Should I dry-pour or mix the concrete first?
Dry-pour is faster and works well for fast-setting mix on a handful of posts. Wet-mixing first tends to give more consistent results on larger posts like 6×6 gate posts.
Do gate posts really need more concrete?
Yes. Gate and corner posts carry more lateral load than straight-line posts, so they typically use a wider hole and noticeably more concrete.
How many bags do I need for a whole fence line?
Multiply your per-post bag count by your total post count, then add extra separately for any gate or corner posts using a wider hole.
Related reading
Get your exact bag count
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