Posts that are too shallow lean and heave; posts that are too deep waste concrete and lumber. The traditional rule is to bury one-third of the post’s total length (so a 6 ft fence needs 3 ft in the ground and a 9 ft post). A lighter rule of one-third of the visible height is common for low fences in firm soil. In cold climates the bottom must be below the frost line.
How to use the Fence Post Depth Calculator
- Fence height above ground
- Rule
- Frost depth (0 if none)
- Gravel base
- Post width
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the fence height, choose the rule, the local frost depth (0 if none), the gravel base and the post width. The calculator gives the burial depth, hole depth, hole diameter and the shortest standard post length.
Inputs explained
| Field | What to enter |
|---|---|
| Fence height above ground | Units: ft, m, in, yd, cm |
| Rule | Choose one: Bury ⅓ of total post length (standard), Bury ⅓ of above-ground height (light duty) |
| Frost depth (0 if none) | Units: in, cm, mm, ft, m |
| Gravel base | Units: in, cm, mm, ft, m |
| Post width | Units: in, cm, mm, ft, m |
What each result means
- Hole depth (incl. gravel) = [Post burial depth] + [Gravel base]
- Hole diameter = 3×[Post width]
- Post length needed = [Fence height above ground] + [Post burial depth]
- Standard post to buy = ceil(([Fence height above ground] + [Post burial depth])÷0.6096 – 0.000001)×0.6096
Example values
How the main result changes when Fence height above ground changes and everything else stays at the example values above:
| Fence height above ground (ft) | Post burial depth (in) |
|---|---|
| 3 | 18 |
| 4.5 | 27 |
| 6 | 36 |
| 7.5 | 45 |
| 9 | 54 |
| 12 | 72 |
Formula
Rule A: buried = ⅓ of post length → depth = height ÷ 2
Rule B: depth = ⅓ of above-ground height
Hole depth = max(rule, frost depth) + gravel base; hole Ø = 3 × post width
Worked example
Inputs: Fence height above ground: 6 ft; Rule: Bury ⅓ of total post length (standard); Frost depth (0 if none): 0 in; Gravel base: 6 in; Post width: 3.5 in.
Results: Post burial depth = 36 in; Hole depth (incl. gravel) = 42 in; Hole diameter = 10.5 in; Post length needed = 9 ft; Standard post to buy = 10 ft.
Frequently asked questions
How deep should a 6 ft fence post be?
About 3 ft (one-third of a 9 ft post), and deeper than the frost line in cold climates.
How wide should a post hole be?
About three times the post width: 10–12 in for a 4×4.
Should fence posts sit on gravel?
Yes, 4–6 in of gravel under the post helps water drain away from the end grain.