Fence post depth is the difference between a fence that still looks straight in ten years and one that starts leaning next summer. A 6 foot privacy fence set 18 inches deep will lean, and it will not take long. The posts are not the expensive part of a fence, but they are the part that decides whether the rest of it was worth building.
Fall is a good time to be thinking about this. The ground is still workable in most of the country, and anything set now has the whole winter to tell you whether you got the depth right.
Here is the arithmetic, plus the two things that override it.
The one third rule
Bury one third of the total post length, then add 6 inches at the bottom for a gravel drainage base.
Work a 6 foot fence through it. A 6 foot fence uses an 8 foot post. One third of 8 feet is 32 inches. Add 6 inches of gravel and the hole is 38 inches deep.
That gravel at the bottom does two jobs. It lets water drain away from the post base instead of pooling there, which meaningfully extends the life of a wood post. And it gives you a level, stable surface to set the post on before any concrete or backfill goes in.
Post depth by fence height
These figures assume reasonable soil, meaning loam or clay with moderate drainage, and a frost line under 36 inches.
| Fence height | Post length | Buried depth | Gravel | Hole depth | Hole diameter |
|---|---|---|---|---|---|
| 3 ft | 5 ft | 20 in | 6 in | 26 in | 8 in |
| 4 ft | 6 ft | 24 in | 6 in | 30 in | 8 in |
| 5 ft | 7 ft | 28 in | 6 in | 34 in | 10 in |
| 6 ft | 8 ft | 32 in | 6 in | 38 in | 10 in |
| 7 ft | 10 ft | 40 in | 6 in | 46 in | 10 in |
| 8 ft | 11 ft | 44 in | 6 in | 50 in | 12 in |
Notice the jump at 7 feet. Posts for fences that tall usually need 10 foot stock, and you are also dealing with considerably more wind load, which is why the buried portion grows faster than the fence does.
Post spacing: 8 feet, with a reason to go shorter
Standard spacing is 8 feet on center, because fence boards, rails and panels are all milled around 8 foot lengths. Six feet on center is the common alternative.
Go to 6 foot spacing when you are building a solid privacy fence in a windy spot, when the ground is uneven enough that an 8 foot rail would visibly sag, or when the soil is sandy and each post carries less than you would like.
The wind math is worth seeing once. A solid 6 foot panel catches roughly 20 pounds of force per square foot in a 60 mph wind. On an 8 foot wide panel that is about 720 pounds of sideways push, and all of it transfers into the posts on either end. Tightening spacing from 8 feet to 6 feet cuts the load each post absorbs by a quarter.
Spacing also sets the rhythm of the finished fence, so it is worth sketching the full run before you dig. The same planning instinct applies to path widths and patio dimensions elsewhere in a yard.
The frost line overrides everything
If your local frost line is deeper than the one third rule gives you, the frost line wins. Always. A post that does not reach below the frost line will be pushed up out of the ground over two or three winter cycles, and no amount of concrete prevents it.
| Region | Typical frost line | Minimum depth for a 6 ft fence |
|---|---|---|
| Deep North (MN, WI, MT, ND) | 48 to 72 in | 54 to 78 in |
| Northeast (NY, PA, MA, CT) | 36 to 48 in | 42 to 54 in |
| Midwest (OH, IN, IL, IA) | 30 to 42 in | 36 to 48 in |
| Mid Atlantic (VA, MD, DE, NC) | 24 to 36 in | 30 to 42 in |
| Mountain West (CO, UT, WY, ID) | 36 to 60 in | 42 to 66 in |
| Pacific Northwest (WA, OR) | 12 to 24 in | 38 in, one third rule governs |
| South (TX, FL, GA, LA) | 6 to 18 in | 38 in, one third rule governs |
| Southwest (AZ, NM, So. CA) | 0 to 12 in | 38 in, one third rule governs |
In Minnesota, a 6 foot fence can need a hole close to 78 inches deep. That is more than 6 feet of digging for a 6 foot fence, and it sounds absurd until you have seen a row of heaved posts in March. Confirm your local number with the building department rather than trusting a regional average.
Concrete or gravel backfill
Both work. Which is better depends on your soil, your fence height and whether you ever want to replace a single post without a demolition hammer.
| Factor | Concrete | Crushed stone |
|---|---|---|
| Drainage | Poor, traps moisture against the post | Excellent, water drains through |
| Ready to load | 4 to 48 hours | Immediately |
| Replacing one post | Hard, you break out the concrete | Easy, dig out the stone and reset |
| Lateral strength | High | Moderate to high, soil dependent |
| Frost heave | Gives frost something rigid to grip | Surrounding soil shifts, post stays |
| Effect on wood post life | Shortens it if not crowned properly | Extends it |
Use concrete for fences over 6 feet, for solid privacy panels in exposed locations, in sandy or loose soil, for every gate post without exception, and wherever local code requires it.
Use crushed stone for fences 6 feet and under, for open styles like split rail and chain link, in soil with decent clay content that compacts well, and when you want the option to move or modify the fence later.
Two details decide whether either method actually works. If you use concrete, crown it 1 to 2 inches above grade and slope it away from the post, because water pooling at the base is the leading cause of early post rot. If you use stone, it has to be angular crushed stone, often sold as 57 stone or crusher run, compacted in 6 inch lifts. Pea gravel is round, so it does not interlock and will shift sideways under load.
Not every post gets the same treatment
This is where most do it yourself fences go wrong. Setting every post at the same depth treats a gate post like a line post, and a gate post lives a much harder life.
- Line posts carry fence boards and wind load. Standard one third rule depth is fine.
- End posts resist the pull of the fence in one direction. Set them 4 to 6 inches deeper than line posts, in concrete.
- Corner posts resist pull from two directions and need the most depth in the whole run, typically 6 to 8 inches deeper than line posts, in a 12 inch diameter hole on taller fences.
- Gate posts take dynamic load every time the gate swings, plus the dead weight hanging off the hinges. Go 6 to 8 inches deeper than line posts, one size larger in post stock, and always concrete.
A single walk gate weighs roughly 35 to 60 pounds. A double drive gate can run 120 to 180 pounds, hanging off one side and swinging several times a day. That is a fatigue load, not a static one, which is why gate posts fail first in almost every fence that fails.
Soil changes the numbers
- Loam. The easy case. Follow the one third rule as written, and either backfill works.
- Clay. Excellent lateral support once compacted, but it holds water against the post. Use a deeper gravel base, 8 to 10 inches instead of 6.
- Sand. The worst case. Poor lateral support, so add about 20 percent to your buried depth. A 6 foot fence wants 44 to 46 inches instead of 38. Use concrete, not stone.
- Rocky. Miserable to dig and excellent once set. If you hit solid rock at 28 inches on a 6 foot fence you are generally fine. Anchor into the rock with a bracket rather than fighting it.
- Expansive soil, found in parts of Texas, Colorado and the Dakotas. It swells wet and shrinks dry, which moves posts seasonally. Use concrete, dig 6 inches deeper than calculated, and flare the footing wider at the base to resist uplift.
Roots are the other ground condition worth planning around. If a large root blocks a post location, move the post 12 to 18 inches rather than cutting through it. Severing a root over 2 inches thick can destabilize the tree, which is a far bigger problem than a post slightly off the line. The reasoning is the same one behind keeping trees a sensible distance from structures in the first place.
Before you dig anything
Call 811 at least three business days ahead. It is free, it is a phone call, and utility locating is the only thing standing between a post hole and a gas or fiber line. In most states, digging without calling makes you liable for the full repair cost, which runs into thousands.
Then check two more things. Your local height limit, because many jurisdictions cap front yard fences at 4 feet and rear yard fences at 6, and many require a permit above 6 feet. And your property line, surveyed if you are not certain, because a fence built even a few inches over is a legal problem that outlives the fence.
Frequently asked questions
How deep should posts be for a 6 foot privacy fence?
Thirty eight inches: an 8 foot post buried 32 inches, over a 6 inch gravel base. If your frost line is deeper than 32 inches, dig to at least 6 inches below it instead. Use concrete for solid privacy panels because of the wind load.
How far apart should fence posts be?
Eight feet on center is standard because lumber and panels are milled to suit it. Drop to 6 feet for solid fences in windy locations, on uneven ground, or in sandy soil.
Can I use gravel instead of concrete?
Yes, for fences 6 feet and under in soil with decent clay content. Use angular crushed stone compacted in 6 inch lifts, never pea gravel. Still use concrete for gate posts, corner posts and anything over 6 feet.
How much concrete does one post take?
For a 6 foot fence with a 10 inch diameter hole and 32 inches of concrete above the gravel, budget about four 50 pound bags of fast setting mix per post. On a 200 foot fence with posts every 8 feet, that is well over 100 bags, so it is worth pricing before you start.
What happens if posts are not deep enough?
They lean within roughly 12 to 18 months from wind load and soil movement, and in cold climates frost heave lifts them out over two or three winters. The repair means digging out the post, re excavating to the right depth and resetting, which is most of the original labor done twice.
The short version
One third of the post buried, plus 6 inches of gravel, unless your frost line says deeper. Eight feet between posts, or 6 feet if the fence is solid and the site is windy. Concrete for gates, corners and anything tall. Crushed stone, never pea gravel, if you go that route. And call 811 before the auger comes out of the truck. Those five rules cover almost every residential fence anyone will ever build.