How Much Concrete per Fence Post?
Quick Answer
A standard 4×4 fence post set in a 10-inch-wide hole, 2 to 2.5 feet deep, needs 1 to 2 80-lb bags of concrete per post. Larger 6×6 posts or deeper holes need 3 or more bags each.
- 4×4 post, 24-inch hole: 1–2 80-lb bags
- 4×4 post, 36-inch hole: 2–3 bags
- 6×6 post, 36-inch hole: ~3 bags
- Rule of thumb: hole depth = 1/3 of above-ground post height
Formula
Calculate the cylinder volume of the hole (π × radius² × depth), then subtract the volume of the post that sits in the hole. An 80-lb bag yields about 0.6 cubic feet of concrete.
Step-by-Step Calculation
- 1
Hole volume (10-in dia, 30-in deep)
3.14 × 0.417² × 2.5 = 1.36 cubic feet
- 2
Post volume (4×4, actual 3.5-in)
0.292 × 0.292 × 2.5 = 0.21 cubic feet
- 3
Concrete needed
1.36 − 0.21 = 1.15 cubic feet
- 4
Bags (80-lb = 0.6 cu ft)
1.15 ÷ 0.6 = ~2 bags per post
Concrete Bags per Fence Post
| Post Size | Hole Diameter | Hole Depth | Concrete | 80-lb Bags |
|---|---|---|---|---|
| 4×4 | 10 inches | 24 inches | 0.92 cu ft | 1–2 bags |
| 4×4 | 10 inches | 36 inches | 1.38 cu ft | 2–3 bags |
| 6×6 | 12 inches | 36 inches | 1.73 cu ft | 3 bags |
| 4×4 (gate post) | 12 inches | 36 inches | 2.10 cu ft | 3–4 bags |
Fast-setting concrete (poured dry into the hole, then water added) is the most common choice for fence posts. One 50-lb bag of fast-set yields about 0.375 cubic feet.
Pro Tips
- Set the post in the hole first, pour dry fast-setting concrete mix around it, then add water — do not pre-mix in a wheelbarrow. The dry-pour method gives you 5–10 minutes of adjustment time to plumb the post before the concrete grabs. One 80-lb bag per post, add 1 gallon of water.
- Dig post holes 3× the post width (12 inches wide for a 4×4 post) for maximum lateral stability. A hole that is too narrow will not resist the wind loads that a 6-foot fence panel catches — fence posts fail from lateral force, not downward pressure.
- In clay soil, flare the bottom of each hole wider than the top (bell shape) — clay contracts and expands with moisture, and a straight-walled hole allows the post to frost-heave upward. The flared base anchors against uplift.
- Leave the concrete crown slightly dome-shaped and sloping away from the post — flat or concave tops pool water at the post base, which is the #1 spot where rot starts on wooden fence posts.
Key Cost Factors
- Fast-setting concrete costs $5.50–$7.00 per 50-lb bag or $6.50–$8.00 per 80-lb bag. Most 4×4 posts need one 80-lb bag ($6.50–$8.00), while 6×6 posts need 2 bags ($13–$16). For a 100-foot fence with 14 posts: $91–$112 for 4×4 posts or $182–$224 for 6×6 posts — concrete is one of the cheapest parts of a fence project.
- Post hole digging is the labor-intensive part. Renting a two-person auger ($50–$80/day) versus hand-digging with a post-hole digger saves 3–4 hours on a 14-post fence. In rocky soil, consider a mini-excavator rental ($200–$300/day) — manual digging in rock is nearly impossible.
Common Mistakes to Avoid
- Making the hole too narrow. A post hole should be about three times the post width. A tight hole leaves too little concrete around the post to resist wind and leaning.
- Digging too shallow. Post depth should be at least one-third the above-ground height and below the frost line. Shallow posts heave in winter and lean over time.
- Filling the bottom with concrete. Concrete poured to the very bottom of the hole traps water against the post end. Add a few inches of gravel first for drainage.
- Not crowning the top. A flat or dished concrete collar holds water at the post base. Slope the top away from the post to shed water and prevent rot.
Related Project Considerations
- Match concrete amount to hole size. A typical 4-inch post in a 10-inch-wide, 24–30 inch-deep hole takes one to two 50-lb bags. Bigger posts and gates need more.
- Consider fast-setting mix for line posts. Pour-in fast-set concrete sets in 20–40 minutes and is convenient for many posts, though heavy gate posts benefit from standard high-strength mix.
- Brace posts while curing. Posts shift before the concrete sets. Plumb and brace each post for at least a few hours, or the whole fence line ends up crooked.
Frequently Asked Questions
How deep should a fence post hole be?
The hole depth should be about one-third of the above-ground post height, plus 6 inches of gravel at the bottom for drainage. For a standard 6-foot privacy fence, dig holes 24 to 36 inches deep and below the frost line in cold climates. Shallow holes are the leading cause of leaning fences — frost heave pushes under-depth posts out of plumb within the first winter in freeze-thaw regions.
Do I need to mix concrete for fence posts?
Not with fast-setting concrete. You can pour the dry mix straight into the hole around the post, then add water — it sets in 20 to 40 minutes with no mixing required. For standard concrete, mix to a stiff consistency before placing it around the post. Either way, crown the concrete above grade so water runs away from the post rather than pooling at the base, which causes premature rot.
How wide should a fence post hole be?
The hole diameter should be about three times the post width. A 4×4 post needs a 10 to 12 inch hole; a 6×6 post needs a 12 to 18 inch hole. Wider holes provide more concrete around the post for lateral stability, which is important in areas with high winds or soft soil. A hole that is too narrow leaves a thin concrete ring that can crack under wind loading on long panel runs.