How Much Concrete for a 4×4 Fence Post?
A 4×4 is the single most common fence post size, and the amount of concrete it needs swings more than people expect based on just two choices: how wide the hole is, and how deep it goes. This guide gives the exact volume — and bag count — for every common combination of the two.
Quick answer
A 4×4 post in a standard 10-inch hole, 2 feet deep, needs about 1.0 cubic foot of concrete — roughly 2 bags of 80 lb mix, once the post’s own volume is subtracted. A narrower 8-inch hole needs closer to half a cubic foot; a wider 12-inch hole for a gate or corner post can need over 2 cubic feet.
For your exact combination of hole size, depth, and post count, the Fence Post Concrete Calculator gives you an instant number.
Volume needed, by hole size and depth
All figures below already have the 4×4 post’s actual volume (3.5×3.5 in) subtracted, plus a 10% waste allowance.
| Hole diameter | 18 in deep | 24 in deep | 30 in deep | 36 in deep |
|---|---|---|---|---|
| 8 in | 0.43 cu ft | 0.58 cu ft | 0.72 cu ft | 0.87 cu ft |
| 10 in | 0.76 cu ft | 1.01 cu ft | 1.27 cu ft | 1.52 cu ft |
| 12 in | 1.15 cu ft | 1.54 cu ft | 1.92 cu ft | 2.31 cu ft |
The same numbers, in bags
| Hole diameter | 18 in deep | 24 in deep | 30 in deep | 36 in deep |
|---|---|---|---|---|
| 8 in | 1 | 1 | 2 | 2 |
| 10 in | 2 | 2 | 3 | 3 |
| 12 in | 2 | 3 | 4 | 4 |
For 60 lb bags, expect roughly a third more bags than the 80 lb column; for 50 lb bags, roughly 60% more.
Why the hole size matters this much
Volume scales with the square of the radius, not straight-line with the diameter — which is why going from an 8-inch to a 12-inch hole roughly triples the concrete needed at the same depth, even though the diameter only grew by half. A wider hole isn’t a small upgrade; it’s a meaningfully bigger pour.
Why the post’s own volume gets subtracted
A 4×4 post genuinely displaces space inside the hole — it isn’t just sitting in empty air. Every figure above already subtracts the post’s actual 3.5×3.5 inch cross-section from the hole’s total volume, which is why the numbers come in lower than a raw hole-volume calculation would suggest. For the full explanation of column versus post math, see Concrete Bags for a Small Column or Post.
The formula behind the tables
0.292 ft (3.5 in) is a 4×4’s actual width — the number that gets squared and subtracted for the post’s own volume.
What to decide before you calculate
- Hole diameter8-10 in for a standard line post; 12 in or more for gate and corner posts.
- Hole depthSet by fence height and local frost depth — see our fence post depth guide for the full rule.
- Bag size80 lb bags are the most cost-efficient per post for most jobs.
- Total post countMultiply your chosen per-post figure by how many you’re setting.
Worked examples
Example 1 — narrow hole, small fence
- Hole8 in diameter, 18 in deep
- Volume needed0.43 cu ft
Just under half a cubic foot — one 80 lb bag covers it with a little room to spare.
Example 2 — standard hole, standard fence
- Hole10 in diameter, 24 in deep
- Volume needed1.01 cu ft
The most common combination for a 6 ft residential fence — right at 2 bags of 80 lb mix.
Example 3 — wide hole, gate post
- Hole12 in diameter, 30 in deep
- Volume needed1.92 cu ft
Nearly 2 cubic feet — 4 bags of 80 lb mix, reflecting the extra stability a gate post needs.
Get your exact number.
Enter your hole diameter, depth, and post count for an instant bag count.
Use the free Fence Post Concrete CalculatorHow many 4×4 posts before ready-mix makes sense
At the standard 10-inch, 2-foot-deep hole (roughly 1 cubic foot per post), it takes about 27 posts to reach one full cubic yard — the point where a ready-mix delivery starts to look more efficient than dozens of individual bags. A typical fence line with fewer than 20-25 posts is squarely bag territory; a long perimeter fence with 30 or more posts is worth a quick ready-mix quote before you commit to bags.
Mistakes specific to 4×4 posts
- Rounding “4×4” to a full 4 inchesThe actual lumber size is 3.5×3.5 in, and using 4 in exactly slightly overstates the post’s displaced volume.
- Using the same hole size for every post on the lineGate and corner posts genuinely need a wider hole than a standard line post.
- Underestimating how much a hole size upgrade changes the totalGoing from an 8 to a 12-inch hole isn’t a small bump — it roughly triples the concrete needed.
- Forgetting to multiply by total post countA per-post number isn’t the project total if you’re setting a dozen or more.
- Rounding a small bag count down1.01 cu ft still rounds up to 2 bags, not down to 1.
What changes the number most
- Hole diameterThe single biggest lever — small increases here have an outsized effect on volume.
- DepthSet by fence height and local frost line, whichever requires more.
- Post roleGate and corner posts typically use a wider hole than standard line posts.
- Bag size chosen80 lb bags need fewer total bags per post than 50 or 60 lb.
Find your number in five steps
- Pick a hole diameter — 8-10 in for line posts, 12 in+ for gate and corner posts.
- Set the depth based on fence height and local frost line.
- Look up the matching cell in the volume table above, or calculate it directly.
- Convert to bags using your chosen bag size’s yield, and round up.
- Multiply by your total post count.
Frequently asked questions
How much concrete does one 4×4 fence post need?
For a standard 10-inch hole, 2 feet deep, about 1 cubic foot — roughly 2 bags of 80 lb concrete mix.
Does hole diameter really matter that much for a 4×4 post?
Yes. Volume scales with the square of the radius, so a modest increase in diameter can nearly triple the concrete needed at the same depth.
How many bags for a 4×4 gate post?
A wider 12-inch hole, 30 inches deep, needs about 4 bags of 80 lb concrete — roughly double a standard line post at the same depth.
Is a 4×4 post’s real size actually 4 inches?
No, a “4×4” measures about 3.5×3.5 inches in reality. Using the nominal 4-inch size instead of the actual size slightly overstates the volume the post displaces.
How many 4×4 posts equal one cubic yard of concrete?
At a standard 10-inch, 2-foot hole (about 1 cubic foot per post), roughly 27 posts add up to one cubic yard — the point where ready-mix starts to look more efficient than bags.
What bag size is best for a 4×4 fence post?
80 lb bags are generally the most cost-efficient, needing fewer total bags per post than 50 or 60 lb sizes.
Do I need less concrete for a 4×4 than a 6×6 post?
Yes, at the same hole size — a 6×6 post displaces significantly more volume, leaving less room for concrete to fill.
Should every post on my fence line use the same amount of concrete?
No. Standard line posts typically use a narrower hole, while gate and corner posts need a wider one and correspondingly more concrete.
Related reading
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