The short answer
For a typical residential fence, bury each post at least one-third of the fence’s above-ground height and never less than 24 inches. Post manufacturers’ instructions go deeper, to about half the above-ground height. Where the ground freezes deeper than that, go to your local frost depth.
Two “one-third” rules, two different answers
| Rule | Measured from | 6 ft fence | Who says it |
|---|---|---|---|
| 1/3 of above-ground height | Visible fence height | 24 in | Angi, Lowe’s (low end) |
| 1/3 of overall post length | Whole post incl. buried part | 36 in | QUIKRETE, Sakrete |
If a third of the post is underground, the above-ground part is two-thirds, so the buried depth equals half the visible height. For a 6 ft fence that is a 9 ft post, bought as 10 ft. The shallower rule fits an 8 ft post. Use the deeper rule for gate, end and corner posts, tall fences, windy sites and loose or sandy soil.
Frost line: when it overrides both rules
Frozen ground expands and can push a shallow post up. Codes require building footings below the frost line (IRC R403.1.4.1); each jurisdiction fills in the frost depth in IRC Table R301.2(1). Minnesota, for example, uses 42 in or 60 in by zone (Minn. R. 1303.1600). Many places do not require a fence permit at all; the model code exempts fences up to 7 ft (IRC R105.2). When your local frost depth is deeper than the rule-of-thumb depth, set the post to the frost depth.
Fence post depth chart (computed by this calculator)
4x4 post, 6 in gravel, 4 in soil cap, no frost override, 24 in floor. Bags: QUIKRETE Fast-Setting 60 lb per hole, 10 in / 12 in hole.
| Fence | Min. embed | Hole | Post | Bags | Rec. embed | Hole | Post | Bags |
|---|---|---|---|---|---|---|---|---|
| 3 ft | 24 in | 30 in | 8 ft | 1.7 / 2.6 | 24 in | 30 in | 8 ft | 1.7 / 2.6 |
| 4 ft | 24 in | 30 in | 8 ft | 1.7 / 2.6 | 24 in | 30 in | 8 ft | 1.7 / 2.6 |
| 5 ft | 24 in | 30 in | 8 ft | 1.7 / 2.6 | 30 in | 36 in | 8 ft | 2.2 / 3.4 |
| 6 ft | 24 in | 30 in | 8 ft | 1.7 / 2.6 | 36 in | 42 in | 10 ft | 2.7 / 4.2 |
| 7 ft | 28 in | 34 in | 10 ft | 2.0 / 3.1 | 42 in | 48 in | 12 ft | 3.2 / 4.9 |
| 8 ft | 32 in | 38 in | 12 ft | 2.4 / 3.6 | 48 in | 54 in | 12 ft | 3.8 / 5.7 |
Worked example
A 6 ft privacy fence where the building department lists a 36 in frost depth, with 4x4 posts: the minimum rule gives 24 in, but frost is deeper, so embed 36 in. The recommended rule gives 36 in too. Hole: 36 + 6 in gravel = 42 in deep, 12 in wide. Post: 6 + 3 = 9 ft, so buy 10 ft 4x4s. Concrete: (π × 6² − 3.5²) × 32 in = 3,227 in³ ÷ 1,728 = 1.87 cu ft ÷ 0.45 = about 4.2 bags per hole.
Common mistakes
- Using “one-third of the post” and “one-third of the fence” interchangeably. They differ by a foot on a 6 ft fence.
- Buying 8 ft posts for a 6 ft fence and then finding that frost depth needs 3 ft in the ground.
- Skipping the gravel base, so the post end sits in water.
- Pouring concrete to the very top instead of stopping 3–4 in below grade.
- Digging before calling 811 to locate buried utilities.
Frequently asked questions
How deep should a 6 ft fence post be?
At least 24 in in the ground (one-third of the height). Manufacturers' post-setting instructions work out to 36 in (half the height). Add 6 in of gravel below either one. If your local frost depth is deeper, go to the frost depth.
How deep should a 4 ft fence post be?
24 in. Both rules fall below the 24 in floor most installation guides use, so 24 in governs unless frost is deeper.
Do fence posts need to go below the frost line?
Codes require building footings to be below the frost line (IRC R403.1.4.1). Fences up to 7 ft are usually exempt from permits (IRC R105.2), but frost heave can still lift a post that stops above the frost line. Ask your building department for the local depth.
Should gate posts be deeper?
Yes. Gate, end and corner posts carry the swinging or pulling load, so use the recommended depth (half the above-ground height) or deeper, and a wider hole.
How wide should a fence post hole be?
About three times the post's actual width: roughly 10–12 in for a 4x4, which is 3.5 in actual (QUIKRETE, Sakrete).
What length post do I need for a 6 ft fence?
An 8 ft post if you bury 24 in. A 10 ft post if you follow the half-height rule or your frost depth is 36 in. Deeper frost needs longer stock: a 60 in frost depth needs 11 ft, so buy 12 ft posts.
Do I put gravel under a fence post?
Yes. Both QUIKRETE and Sakrete call for a compacted gravel base (4–6 in) under the post so water drains away from the end grain.