How deep should fence posts be?
Bury one third of the post, or go below the frost line — whichever is deeper.
For a 6 ft fence on an 8 ft post that is 24 to 32 in in mild ground. In Minnesota, North Dakota or Alberta the frost line overrules it and you are digging 60 in or more.
The two rules, and which one wins
The one-third rule is about leverage. A fence is a sail, and wind load tries to lever the post over; a third of its length in the ground gives enough embedded depth to resist it. On an 8 ft post that is 32 in, leaving 64 in above grade for a 6 ft fence and a little trim.
The frost line is about a different failure entirely. Wet soil expands as it freezes, grips the sides of the post and lifts it — and it does not put it back down. Do it three winters running and the posts stand proud, the rails rack, and no amount of concrete helps, because the concrete is what the frost is gripping. The fix is to end the footing below the depth that freezes, where the ground never moves.
They are not alternatives. Work out both and dig to the deeper one.
Frost depth where you are
| Location | Typical frost depth | Minimum post depth |
|---|---|---|
| Minnesota | 60–80 in | 80 in |
| North Dakota | 60–80 in | 80 in |
| Maine | 48–72 in | 72 in |
| Wisconsin | 48–60 in | 60 in |
| Alberta (Calgary) | 48–72 in | 72 in |
| Ontario | 48–60 in | 60 in |
| Michigan | 42–48 in | 48 in |
| New York | 36–48 in | 48 in |
| Illinois | 40–48 in | 48 in |
| Colorado | 30–36 in | 36 in |
| Indiana | 32–36 in | 36 in |
| Ohio | 32–36 in | 36 in |
| Missouri | 24–30 in | 30 in |
| Virginia | 18–24 in | 24 in |
| Tennessee | 12–18 in | 24 in |
| Texas (north) | 6–12 in | 24 in |
| Georgia | 5–10 in | 24 in |
| Florida | 0–0 in | no frost requirement |
These are representative figures, not code. Frost depth is set locally and can differ by two feet across one state — Minnesota's northern counties are nearly that far from its southern ones. Your building department will give you the number for your address in one phone call, and it is the single number worth confirming before you dig, because it is the one you cannot correct later.
Where there is no frost requirement, the one-third rule governs and 24 in is the practical floor for anything carrying a panel.
What depth actually costs
Depth is cheap while you are digging and expensive afterwards. Going from 24 in to 36 in on a 4x4 adds about 0.5 cu ft of concrete per post — 1 more 80 lb bags — and on a 30 post fence that is 30 bags. Real money, and still less than rebuilding a heaved fence.
Common questions
- How deep for a 6 ft fence?
- Use an 8 ft post and bury 24 to 32 in, unless your frost line is deeper. The common mistake is buying 6 ft posts for a 6 ft fence, which leaves nothing to bury.
- Does concrete stop frost heave?
- No — a concrete plug above the frost line gives the ice something bigger to grip, so it can heave harder. Depth is what stops heave. Belling the bottom of the hole wider than the top helps too, because then the plug cannot be pulled straight up.
- Do I need concrete at all?
- Not always. Well-tamped gravel drains better and lets a rotten post be replaced without jackhammering, and for farm and stock fence it is standard. Concrete wins for panel fences, gates and anywhere the wind load is high.
- How deep in Wisconsin, Michigan or Minnesota?
- Deeper than the one-third rule would ever suggest. Wisconsin typically 48–60 in, Michigan 42–48, Minnesota 60–80. In that ground the frost line is the only number that matters, and it is why fences built to a generic 24 in guide fail there in a few seasons.
- What about Ontario or Calgary?
- Ontario is commonly 48–60 in and Alberta 48–72. Both are frost-governed, and both have municipal figures that override any table — including this one.
- Does the hole need to be wider at the bottom?
- It helps a great deal in frost country. A hole belled out at the base makes a footing wider than its neck, which frost cannot lift without shearing the concrete. It is the cheapest insurance on this whole list.