⅓ rule vs. frost line
Two rules compete for the bottom of every post hole. The deeper one wins. This does both in one pass — frost line included.
| Dig | Figure |
|---|
Hole depth includes 6″ of gravel under the post for drainage.
The ⅓ rule is about leverage: a third of the post underground keeps the wind from using your fence as a sail. The frost line is about physics underground: freeze the soil under a post base and it lifts — an inch a winter, until the gate won't latch. Leverage you can argue with. Frost you can't. Find your state's line here and this page fills it in automatically.
One third of the height above ground, never less than 24″, and never shallower than your frost line — the deepest of those three wins. A 6 ft privacy fence in frost-free country wants 24″; the same fence in Minnesota wants the frost line's 42–60″.
They carry the pull. A gate slams and sags on its post daily, and corner posts take tension from two directions of fence. The extra 6″ of burial is the cheapest rigidity you'll ever buy.
Drainage. Six inches of gravel under the post keeps the end grain out of standing water, which is what actually rots posts. Concrete around, gravel below — each doing its own job.
Concrete for gates, corners, ends, and anything in wind or frost country; well-tamped gravel is a legitimate choice for line posts in stable soil and drains better. This site does the concrete math either way — the hole size is the same.