Setting fence posts? Here is how many bags of concrete each post needs, for 4×4 and 6×6 wood posts and round chain-link posts, plus the hole size to dig and a free calculator in inches.
| 4×4 post, 6 ft fence | 2 × 80 lb or 3 × 50 lb fast-setting |
|---|---|
| 6×6 post, 8 ft fence | 5–8 × 80 lb (hole size matters) |
| Chain link line post (2⅜ in) | 2 × 50 lb fast-setting |
| Hole diameter | About 3× the post width |
| Hole depth | About ⅓ of fence height, plus 6 in gravel |
How big should a fence post hole be?
- Diameter: about 3 times the width of the post. A 4×4 (actual 3½ in) needs a 10–12 in hole; a 6×6 (actual 5½ in) needs 16–18 in.
- Depth: about one-third of the post height above ground, and at least 24 in. A 6 ft fence needs about 24 in of post in the ground; an 8 ft fence about 32 in.
- Gravel: add about 6 in of gravel at the bottom so water drains away from the end grain of wood posts.
- Frost line: in cold climates, take the hole below the local frost depth so frost heave does not lift the post. Your building department can tell you the depth.
- Gates and corners carry more load. Use the larger hole size there.
Concrete per fence post chart
| Post | Hole (diameter × depth) | Concrete per post | 50 lb fast-setting bags | 80 lb bags |
|---|---|---|---|---|
| 4×4 wood (3.5 in) | 10 in × 30 in | 0.92 ft³ | 3 | 2 |
| 4×4 wood (3.5 in) | 12 in × 30 in | 1.40 ft³ | 4 | 3 |
| 4×4 wood (3.5 in) | 10 in × 36 in | 1.15 ft³ | 4 | 2 |
| 6×6 wood (5.5 in) | 16 in × 36 in | 2.97 ft³ | 8 | 5 |
| 6×6 wood (5.5 in) | 18 in × 42 in | 4.67 ft³ | 13 | 8 |
| 2⅜ in steel (chain link line post) | 8 in × 30 in | 0.64 ft³ | 2 | 2 |
| 2⅜ in steel (chain link line post) | 10 in × 36 in | 1.29 ft³ | 4 | 3 |
How the calculation works
- Concrete height = hole depth − gravel depth
- Hole volume = 3.1416 × (diameter ÷ 2)² × concrete height (cubic inches)
- Subtract the post: post width × post thickness × concrete height (or 3.1416 × radius² for a round post)
- Divide by 1,728 to get cubic feet
- Divide by the bag yield (0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb, 0.375 ft³ for 50 lb fast-setting) and round up
Worked example: 20 posts for a 6 ft privacy fence
- Post: 4×4 (3½ in) for a 6 ft fence. Guideline hole: 10 in wide × 30 in deep (24 in of post in the ground plus 6 in of gravel)
- Concrete height = 30 − 6 = 24 in
- Hole = 3.1416 × 5² × 24 = 1,885 in³; post = 3.5 × 3.5 × 24 = 294 in³
- Concrete = (1,885 − 294) ÷ 1,728 = 0.92 ft³ per post
- 80 lb bags: 0.92 ÷ 0.60 = 1.53, so 2 bags per post, 40 bags for 20 posts
- Or 50 lb fast-setting: 3 bags per post, 60 for 20 posts
- Gravel: about 5.5 ft³ for all 20 holes
Fast-setting or regular concrete for posts?
| Fast-setting (50 lb) | Regular mix (60/80 lb) | |
|---|---|---|
| How you use it | Pour dry into the hole, then add water | Mix in a tub or wheelbarrow first |
| Sets in | About 20–40 minutes | Several hours |
| Concrete per bag | Less (≈0.375 ft³) | More (0.45–0.60 ft³) |
| Best for | Fence posts, mailboxes, quick jobs | Gate posts, many holes, lower cost per ft³ |
How to set a fence post in concrete
- Call 811 before you dig so buried utility lines are marked.
- Dig the hole to full depth, slightly wider at the bottom than the top.
- Add about 6 in of gravel and tamp it.
- Set the post, then brace it plumb in two directions.
- Fill with concrete to just below ground, then slope (crown) the top away from the post so water runs off.
- Let it cure as the bag directs before attaching rails or gates.
FAQ
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 in × 30 in hole with 6 in of gravel, about 2 bags of 80 lb concrete or 3 bags of 50 lb fast-setting mix. A 6×6 post needs about 5 bags of 80 lb in a 16 in × 36 in hole.
How deep should a fence post be?
About one-third of the height above ground, and at least 24 in, plus about 6 in of gravel. Go below the frost line in cold climates.
How wide should a fence post hole be?
About 3 times the post width: 10–12 in for a 4×4 and 16–18 in for a 6×6.
Can I use fast-setting concrete for fence posts?
Yes. Pour it dry into the hole and add water as the bag directs. It sets in roughly 20–40 minutes but each bag fills less volume than regular concrete.
How many 50 lb bags of fast-setting concrete per post?
About 3 for a 4×4 post in a 10 in × 30 in hole, and about 8 for a 6×6 post in a 16 in × 36 in hole.
Hole sizes are common US installer guidelines; bag yields are typical values printed on US bagged concrete. Local codes, frost depth and wind loads can require larger or deeper holes.
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