Post depth is the single biggest factor in whether a fence stays straight or leans within a couple of seasons. Set the posts too shallow and frost, wind and the weight of the fence itself will work them loose. This guide covers the standard rule, the two things that override it, and how to work out the concrete each hole needs.
The rule of thumb
The common guideline is to bury at least one-third of the post’s total length, or a minimum of 2 feet deep — whichever is greater.
So for a 6-foot-tall fence you want roughly 2 feet in the ground, which means buying 8-foot posts (6 feet above grade, 2 feet below). For an 8-foot post you’d set it about 32 inches deep. Deeper is almost always better for stability; shallower is asking for a lean.
Two things that override the rule
The one-third rule is a floor. Two local factors can push the depth deeper:
1. Frost line
In cold climates the ground freezes and heaves each winter. If the bottom of your post sits above the frost line, that heave can lift and tilt it over time. Post holes should extend below the local frost line — which can be 36 inches or more in northern regions. Your local building department publishes the frost depth for your area; when it’s deeper than the one-third rule, the frost line wins.
2. Fence height and wind load
A tall privacy fence is a big sail. The taller and more solid the fence, the more leverage the wind has on each post, so tall or solid fences on exposed sites benefit from deeper, wider holes than the minimum. Corner and gate posts take the most load and are often set deeper still.
How wide, and how much concrete?
A good hole is about three times the post’s width in diameter — roughly an 8- to 12-inch hole for a standard 4x4 post — and it should flare slightly wider at the bottom to resist uplift. Add a few inches of gravel at the base for drainage so water doesn’t pool and rot the post.
The concrete you need per hole depends on the hole diameter, its depth, and the post taking up space in the middle. Rather than guess how many 60- or 80-pound bags to buy, plug your hole size and post count into the Fence Post Concrete Calculator — it does the volume math and rounds up to whole bags so you don’t come up short on the last few posts.
Setting posts that last
- Dig below the frost line if you’re in a cold climate, even if that’s deeper than the one-third rule.
- Add gravel at the bottom for drainage before you set the post.
- Slope the concrete slightly above grade so water runs away from the post, not into it.
- Brace and check for plumb while the concrete cures — a post that sets crooked stays crooked.
- Set terminal, corner and gate posts deeper than line posts, since they carry the most stress.
Get the depth and concrete right up front and the rest of the fence is easy. Get it wrong and no amount of good pickets will keep the line from sagging.
Depth guidance here is general — always check your local building code and frost-depth requirements before digging.