How deep should fence posts be?

The first time you set a fence post wrong, you find out at month nine. The post that looked plumb in October is leaning west by June, and the picket line has the wave you only ever see on neighbors' fences. Depth is the cheapest part of a fence to get right and the most expensive to get wrong.

Below is the way working fence crews actually decide post depth — frost line first, then height, then soil, then gate logic. There's a calculator about halfway down so you can plug in your ZIP and skip the reading if you'd rather. But the reading is shorter than you'd think.

The short answer.

Set a residential fence post at least one-third of the post's above-grade height, and at least 6 inches below the local frost line, whichever is deeper. For most of the United States, that means a hole between 30 and 48 inches deep, 10 to 12 inches in diameter, with one to two 80-lb bags of fast-set concrete per post and 3–6 inches of drainage gravel at the bottom.

Corner posts and gate posts go deeper. Sandy soil goes deeper. Clay goes shallower but wider. If you remember nothing else: a fence fails at its corners and its gates — those are the posts to over-build.

The one-third rule, and why it isn't enough.

You'll see this rule everywhere: a third of the post should be in the ground. For an 8-foot post supporting a 6-foot fence, that's 2 feet of post below grade, with 6 feet showing. It's a fine starting point — and it's also why your neighbor's fence is doing what it's doing.

The one-third rule is a structural rule. It assumes the post itself isn't moving. But in most of the country, the ground moves: water freezes, expands, and lifts everything sitting in it. The rule that beats one-third is called the frost line, and it's local.

This is why a fence in Phoenix is set differently from a fence in Burlington, Vermont, even if both are 6 feet tall. Phoenix has effectively no frost line; the one-third rule wins. Burlington's frost line is 60″+; the frost rule wins by a mile.

Frost line, by ZIP.

The frost line (or frost depth) is the maximum depth at which the ground freezes in winter. Anything not anchored below it gets pushed up and out by frost heave. Frost heave doesn't break posts — it lifts them, lets gravity drop them off-center, and over a few cycles, your fence develops the wave.

The “Plumbline default” column adds a 6-inch safety margin and bumps everything below code minimum up to a constructible depth — fence posts can be set to 12″ in Florida, but most pros set 24″ anyway because it gives the post real lateral stability against a Cat 1 hurricane.

Frost heave doesn't break posts. It lifts them, lets gravity drop them off-center, and over a few cycles, your fence develops the wave you see on every neighbor's fence.

What the soil is actually doing.

Two posts of identical depth in different soils behave like different posts. The post is a lever; the soil is the fulcrum; and the fulcrum has different stiffness depending on what it's made of.

Clay loam

Stiff when wet, stiffer when dry. Clay grips a post like a hand. The risk is expansion: clay swells when wet and shrinks when dry, which over years cracks an over-poured concrete collar. Plumbline's default in clay: standard depth, wider footing (12″), gravel at the base for drainage.

Sandy / sandy loam

Drains fast — good for the post, bad for the grip. Sand doesn't hold lateral load. Plumbline's default in sand: add 6 inches of depth, oversize the footing diameter to 12–14″, and skip gravel (it's already drainage).

Rocky / glacial till

The hardest to dig and the easiest to set. Once you've fought a hole into rocky soil, the soil itself is structural. Use the code minimum, no concrete required if you're driving a metal post, and pack with crushed gravel.

Line vs. corner vs. gate.

The single most common DIY mistake is treating every post identically. A residential fence has three structural roles, and each one wants a different hole.

1. Line posts — the bulk of the fence, doing axial load only. These follow the standard rule: ⅓ of height, frost + 6″, 10″ diameter.
2. Corner & end posts — these resist two rails pulling in different directions, plus the entire fence's wind load translated to a moment arm at the corner. Plumbline adds +6″ depth and a 12″ diameter hole at every corner, with a heavier 6×6 post in place of the typical 4×4.
3. Gate posts — the worst job in the fence. A 4-foot walk gate hangs ~80 lb, plus dynamic load from kids slamming it; a 10-foot drive gate is 200 lb of cantilever swinging on two hinges. Plumbline sets gate posts +6″ deeper than corners, in 14″ diameter footings, with two bags of concrete.

In Westport, CT, where the Plumbline demo project lives — frost line 42″, clay loam, 6-foot cedar privacy with a pool gate and a drive gate — that comes out to: line posts at 48″, corners at 54″, gate posts at 60″.

Footing diameter & concrete math.

Footing diameter matters more than most DIYers think. Three times the post's nominal width is the rule — a 4×4 post wants ~12″, a 6×6 wants ~18″. For volume: an 80-lb bag of fast-set Quikrete pours about 0.6 cubic feet. A 48″ × 10″ hole, minus the post displacement, takes about 1.4 bags. Round up. Always round up.

Add 10% for waste, spillage, and the bag that always cracks open in the truck. A 456-foot perimeter fence with 25 line posts, 6 corners, and 4 gate posts comes out to ~76 bags of fast-set, which is what the Plumbline demo project shows in the BOM.

Five mistakes pros never make.

• Pouring a domed concrete collar. Concrete should slope away from the post at grade so water sheds. A flat or domed-inward collar pools water at the post and rots it from the bottom up.
• Skipping gravel at the base. 3–6″ of crushed gravel at the bottom of the hole gives the post end a way to dry out. Without it, the bottom of a wood post sits in standing water for half the year.
• Mixing concrete too wet. Fast-set is designed for the dry-pour method — pour the bag in dry, add water on top.
• Setting all posts to the same depth. Corners and gates do different work. The fence will fail at whichever post is under-set first.
• Setting posts before the rails are dry-fit. The professional sequence: dig all holes, drop posts loose, dry-fit one full run of rail to confirm spacing, then pour. Concrete is the last step, not the first.

Field FAQ.

Can I use gravel instead of concrete?

For a 4-foot split-rail or chain link, yes — packed crushed gravel performs as well as concrete and lets the post dry out. For 6-foot privacy, no: the wind load on a solid privacy panel needs concrete to anchor against.

Does the post have to be pressure-treated below grade?

Yes, always. Look for a 0.40 ACQ retention or higher (sometimes labeled “ground contact”). Standard above-ground PT (0.15) will rot in soil within five to seven years.

How long until I can hang the fence?

Fast-set concrete is structural at 4 hours and fully cured at 24. Most pros pour the posts on day one, come back the next morning, and start the rails. If you're using regular Quikrete (not fast-set), wait 48 hours.

Do I need a permit?

It depends on your municipality and the fence height. Most places require a permit for anything over 6 feet, anything in a front yard, and anything that touches a property line where a survey is unclear. Plumbline auto-flags permit requirements based on your ZIP and fence spec.