Post-hole depth reference — how deep should fence posts be?
A labeled planning reference for fence post depth: buried depth, gravel base, total post length and hole diameter from the above-ground height.
Calculator
A 6 ft above-ground fence typically sets about 24" deep (1/3 of the height) plus a 6" gravel base — a 30" hole and a total post ≈ 8.5 ft. These are labeled planning values — frost depth, soil, wind load and local code set the real depth; call 811 before you dig.
A fence post only stands up to wind and leaning if enough of it is in the ground. The common planning guideline is to bury about one-third of the above-ground height, then add a few inches of gravel under the post for drainage. So a 6 ft fence sets roughly 2 ft (24") deep, over a 6" gravel base, for about a 30" hole and a total post around 8.5 ft.
The hole diameter follows a separate rule of thumb: about three times the post width, which gives enough concrete ring to hold the post. Size the concrete for that hole with the concrete-per-post calculator.
This is a labeled planning reference, not a structural or geotechnical design. Frost depth, soil type, wind load and local building code set the real depth and footing — a shallow post above the frost line can heave. Check your local building department, and call 811 to locate buried utilities before you dig. A licensed engineer sizes load-bearing or high-wind posts.
Formula
buried_depth = post_height_ft ÷ 3 (≈ one-third of the height)\nhole_depth = buried_depth + 6" gravel base\ntotal_post = post_height_ft + buried_depth + 0.5 ft\nhole_dia ≈ 3 × post_width_in
Worked example
A 6 ft privacy fence on nominal 4×4 posts:
- buried ≈ 6 ÷ 3 = 2 ft = 24"
- hole = 24" + 6" gravel = 30"
- total post ≈ 6 + 2 + 0.5 = 8.5 ft
- hole diameter ≈ 3 × 4 = 12"
Reference table
Planning depth by above-ground fence height (one-third rule + 6" gravel):
| Fence height | Buried depth | Hole depth |
|---|---|---|
| 3 ft | 12" | 18" |
| 4 ft | 16" | 22" |
| 5 ft | 20" | 26" |
| 6 ft | 24" | 30" |
| 8 ft | 32" | 38" |
Labeled typicals — frost depth, soil, wind load and local code set the real depth. Call 811 before you dig.