A standard 4×4 line post needs 1–2 bags of 80-lb concrete per hole (at a 10-inch diameter, 24-inch deep hole). A 6×6 gate or corner post at a larger hole depth needs 3–5 bags. The actual volume depends entirely on hole diameter and depth — use the formula below for your specific dimensions.
How to Calculate Concrete Volume for a Post Hole
Post holes are cylinders. The formula:
Volume (cu ft) = 3.14159 x (diameter in ft / 2)² x depth (ft)
Subtract the post volume to get net concrete needed: Post displacement (sq ft) = post width x post width / 144 x depth (ft)
Reference Table: Bags per Post Hole
| Hole Diameter | Hole Depth | Post Size | Hole Volume | Net Concrete | 80-lb Bags | 60-lb Bags |
|---|---|---|---|---|---|---|
| 8 in | 24 in | 4x4 | 0.70 cu ft | 0.68 cu ft | 1–2 | 2 |
| 10 in | 24 in | 4x4 | 1.09 cu ft | 1.07 cu ft | 2 | 3 |
| 12 in | 24 in | 4x4 | 1.57 cu ft | 1.55 cu ft | 3 | 4 |
| 12 in | 36 in | 4x4 | 2.36 cu ft | 2.33 cu ft | 4 | 6 |
| 16 in | 36 in | 6x6 | 4.19 cu ft | 4.08 cu ft | 7–8 | 10 |
| 18 in | 36 in | 6x6 | 5.30 cu ft | 5.20 cu ft | 9 | 12 |
Note: Add 10% for waste. Always round up to the nearest whole bag.
Step-by-Step Worked Example
You are setting a 6-foot privacy fence with 8-foot 4×4 posts (2 ft in ground). The hole specification calls for a 10-inch diameter hole, 24 inches deep, with 2 inches of gravel at the bottom (so 22 inches of concrete above the gravel).
Step 1: Convert to feet: diameter = 10/12 = 0.833 ft; concrete depth = 22/12 = 1.833 ft
Step 2: Hole volume = 3.14159 x (0.833/2)² x 1.833 = 3.14159 x 0.174 x 1.833 = 1.00 cu ft
Step 3: Post displacement = (3.5/12)² x (22/12) = 0.085 x 1.833 = 0.016 cu ft
Step 4: Net concrete = 1.00 − 0.016 = 0.98 cu ft per post
Step 5: 80-lb bags: 0.98 / 0.60 = 1.64 → add 10% → 2 bags per post
For a 100-foot fence at 8-foot spacing: approximately 14 posts x 2 bags = 28 bags of 80-lb concrete
Use the concrete calculator to enter your exact hole dimensions and post count for a complete bag estimate.
Proper Post Depth by Fence Height
| Fence Height | Recommended Post Length | Min. Hole Depth |
|---|---|---|
| 3–4 ft | 6 ft | 2 ft (24 in) |
| 5–6 ft | 8 ft | 2.5 ft (30 in) |
| 7–8 ft | 10 ft | 3 ft (36 in) |
Add 6 inches to the hole depth for a gravel drainage layer at the bottom. This keeps water from pooling around the base of the post and significantly slows wood rot. For more on post depth requirements, see fence-post-depth.
In cold climates (USDA Hardiness Zones 3–6), frost heave can push posts out of the ground if they are not set below the local frost depth. Check your county’s frost depth map — in Minnesota that can be 42–60 inches. Setting posts to frost depth prevents heaving regardless of concrete or tamped soil.
Call 811 Before Digging
Before digging any post hole, call 811 (the national call-before-you-dig hotline) at least three business days in advance. Underground utilities — gas, electric, water, cable — are present in most yards and are not always where you expect them. The service is free and legally required in all US states.
Fast-Setting Concrete for Fence Posts
Quikrete Fast-Setting Concrete (#1004) is designed for post-setting. Pour the dry mix directly into the hole around the post, then add water slowly on top. No pre-mixing is needed. It sets in 20–40 minutes at 70°F and reaches adequate strength in 4 hours.
An 80-lb bag of Quikrete Fast-Setting yields 0.60 cu ft — the same volume as their standard mix. The difference is convenience and speed, not yield.
Setting Posts Without Concrete
Concrete is not always required. For light-duty applications — split-rail fences, garden borders, or low-traffic areas — tamped soil or gravel can work if:
- The post is pressure-treated wood rated for ground contact (UC4B or UC4C)
- The native soil is dense, not sandy or expansive clay
- The fence will not experience gate swing forces or high wind loads
The drawback of soil-set posts is faster decay at grade level — water and soil microbes attack wood most aggressively at the soil line. Concrete collars prevent direct soil contact and extend post life. The concrete should be sloped or crowned at the top so water drains away from the post rather than pooling.
Post material and rot resistance:
| Post Material | Typical Lifespan (in ground) | Notes |
|---|---|---|
| Pressure-treated pine (UC4B) | 20–30 years | Standard for wood fencing |
| Cedar (untreated) | 15–20 years | Better than pine; still rots |
| Black locust (untreated) | 30–50 years | Naturally rot-resistant |
| Steel galvanized post | 30–50 years | No concrete needed with bolt-in anchors |
| Composite post | 25–40 years | Manufacturer-dependent |
Concrete Crown vs. Flush Set
A long-running debate among fence builders is whether to crown the concrete (dome it above grade) or set it flush. Each has tradeoffs:
- Crowned above grade: Sheds water away from the wood post. But the crown is exposed and can crack and collect water over time.
- Flush with grade: Easier to mow around. Some water may channel toward the post base.
- Set 2 inches below grade and backfill: Soil covers the concrete; the post wood is still protected by the concrete collar. The buried concrete is not exposed to freeze-thaw cycles, so it lasts longer.
Many professional fence installers prefer the below-grade set for longevity. The concrete still provides the structural anchor; the soil covering protects it.
Safety Note
Wet concrete is alkaline (pH 12–13). Prolonged skin contact causes chemical burns. Wear nitrile or rubber gloves when mixing or handling any concrete product. If concrete splashes into eyes, flush with clean water for 15 minutes and seek medical attention.