For a standard 6-foot wood privacy fence, set posts 24 to 36 inches deep in a concrete footing, with the specific depth determined by your local frost line and any city ordinances. The widely cited one-third rule (bury one-third of total post length) is a starting point, not a code requirement — your local frost depth and soil type are what actually govern.
The One-Third Rule
Bury one-third of the total post length below grade. A fence built on 8-foot posts at 6 feet above grade goes in 2 feet (24 inches). A 9-foot post for a 6-foot fence goes in 36 inches. A 12-foot post for an 8-foot fence goes in 48 inches.
| Fence height | Common post length | 1/3 rule depth |
|---|---|---|
| 4 ft | 7 ft | 28 in. |
| 6 ft | 9 ft | 36 in. |
| 8 ft | 12 ft | 48 in. |
ASTM F567, the chain-link fence installation standard reproduced by manufacturers for residential use, sets a minimum depth of 24 inches plus 3 inches for each foot of fence height above 4 feet. That puts a 6-foot fence at 30 inches minimum — lower than the one-third rule on a 9-foot post.
Frost Line: The Number That Overrides Everything
In cold climates, the bottom of the concrete footing must sit below the local frost line. Frozen soil expands and exerts upward force on anything embedded in it — a process called frost heave. Posts set above the frost line will shift upward over freeze-thaw cycles, racking the fence and loosening the concrete collar.
Frost line depth varies enormously by location:
| Region | Approximate frost depth |
|---|---|
| Florida, Gulf Coast | 0–6 in. |
| Tennessee, North Carolina | 6–12 in. |
| Virginia, Ohio, Indiana | 24–32 in. |
| Minnesota, Wisconsin | 42–60 in. |
| Parts of Montana, Colorado | 40–50 in. |
These are approximate ranges. IRC Table R301.2 intentionally leaves frost depth blank and instructs each jurisdiction to fill it in — there is no single national number. Your city’s building department publishes the frost depth for your area. Look it up before you dig.
Local Ordinances Can Override the Frost Line Too
Some cities mandate a minimum depth regardless of frost line. Requirements vary widely — some jurisdictions tie depth to frost line tables, others set a flat minimum (commonly 36 to 42 inches in cold climates, 24 to 30 inches in moderate zones). A few require a permit and inspection before the concrete pour.
Call your building department before you dig. Post depth is the one number that can fail an inspection when a permit is involved. Ask specifically: what is the minimum post embedment depth for a residential wood fence in my zone?
Hole Diameter
The hole should be about three times the post width:
| Post size | Actual width | Hole diameter |
|---|---|---|
| 4x4 | 3.5 in. | 10–12 in. |
| 6x6 | 5.5 in. | 16 in. |
| 4-in. round steel | 4 in. | 12 in. |
Wider holes are needed in sandy or loose soil to distribute lateral load over more surface area.
How Much Concrete Per Post
A 10-inch hole dug 36 inches deep holds about 0.21 cubic feet of concrete per inch of depth — approximately 2.0 cubic feet total. At 0.45 cubic feet per 60-lb bag of ready-mix, that is roughly 4.5 bags per hole. Use the fence calculator to total up concrete for your post count, or see concrete for fence posts for detailed per-post calculations.
Setting the Post
- Pour 4 inches of gravel in the bottom of the hole for drainage. Gravel at the base prevents water from pooling at the post end, which is where rot starts.
- Set the post plumb (check with a level on two perpendicular faces).
- Brace the post so it stays plumb while concrete cures.
- Pour concrete and rod it to remove air pockets.
- Slope the concrete collar away from the post — do not leave it flat or cupped, which traps water.
- Let cure for 24–48 hours before attaching panels.
Wood Post Preservation
Pressure-treated lumber rated for ground contact (UC4A or UC4B) is required for posts that will be embedded in soil or concrete. The treatment designation matters: UC3 is rated for above-ground use only. Look for the tag on the end of the board — it lists the retention level and use category.
Keep the cut end of any pressure-treated post away from the soil when possible. If you cut a post, apply a brush-on preservative to the cut end before setting.
Difficult Soil Conditions
Standard concrete-footing installation assumes firm, stable soil. Three conditions require a different approach:
Rocky soil. When you hit solid rock at depth, ASTM F567 (Section 5.5) specifies: drill a hole with diameter equal to the post’s largest cross section plus 1/2 inch, and depth equal to three times the post’s largest cross section. Set the post in nonshrinkable hydraulic cement, not standard concrete. This creates a bonded mechanical connection in rock that outperforms a concrete collar in poor soil.
Unstable or sandy soil. Loose, cohesionless soil offers little passive resistance to lateral forces. Per ASTM F567 Section 5.9, increase embedment depth by 20% over standard depth in unstable soil, or use driven posts rather than concrete-set posts. Widening the hole and pouring more concrete helps, but in very loose sand, driven posts (mechanical post drivers) often provide better lateral resistance than concrete footings.
Expansive clay soil. Clay that swells when wet can exert enough upward force to lift a post regardless of depth. In highly expansive clay regions — common in Texas, Oklahoma, and parts of the Southeast — post sleeves or helical pier anchors can outperform concrete collars by allowing the sleeve to move without transferring force to the post.
Drive Posts vs. Concrete-Set Posts
Metal T-posts and some steel fence posts are installed by driving rather than in concrete. ASTM F567 Section 5.8 requires driven posts to reach a minimum depth of 36 inches (914 mm), or 6 inches deeper than the depth calculated for a concrete-set post of the same fence height. Drive posts require firm, penetrable soil — rocky ground stops a post driver and requires switching to concrete-set installation.
For spacing posts correctly across your fence run, see fence post spacing. For full project cost, see fence cost per foot.