Fence Post Depth Calculator
Enter your fence height and post width to find how deep to set each post, the post length to buy, the hole diameter, and roughly how much concrete you'll need.
A fence is only as sturdy as its posts, and the posts are only as sturdy as their footing. The classic rule of thumb is to bury one-third of the post — so for every foot of fence above ground you set half a foot below it — with a minimum of two feet. This calculator applies that rule and sizes the hole and concrete for you.
How deep should fence posts be?
Set the buried depth to one-third of the total post length, which works out to half the above-ground height, and never less than 2 feet. The hole should be about three times the post's width so there's room for concrete all around.
Post length = Fence height + Buried depth
Hole diameter = Post width × 3
| Fence height | Buried depth | Post length |
|---|---|---|
| 4 ft | 2 ft (minimum) | 6 ft |
| 6 ft | 3 ft | 9 ft |
| 8 ft | 4 ft | 12 ft |
Worked example
A 6 ft fence with 4×4 posts:
| Buried depth = 6 ÷ 2 | 3 ft |
| Post length = 6 + 3 | 9 ft |
| Hole diameter = 4 × 3 | 12 in |
Frost line and soil matter
In cold climates the ground freezes and heaves, so posts must be set below the frost line — sometimes 3–4 feet or more, deeper than the one-third rule. Check your local building department for the frost depth. Sandy or loose soil also calls for a wider hole and more concrete, while gate posts and corner posts should go deeper than line posts because they take more load.