Fence post depth calculator

How this calculator works

A fence post has to resist two things: wind pushing the fence over, and frost pushing the post up. Each sets its own minimum depth, and the calculator uses whichever is deeper.

Embed depth = max(fence type minimum, frost depth + 6 in)

The fence type minimum comes from the height of the fence. The common rule is to bury about one third of the post's total length, which works out to about half of the height above ground. A 6 ft fence wants about 3 ft in the ground.

The frost rule says the bottom of the footing must sit at least 6 in below the depth where your ground freezes. Frozen soil expands and grips the concrete, and a post that stops inside the frost zone gets lifted a little every winter. Going 6 in below it puts the base in soil that does not move.

Gate and corner posts go 6 in deeper. A gate post carries a swinging load and a corner post carries tension from two directions, so both get extra depth over the line posts.

Post length = height above grade + embed depth, rounded up to the next stock length of 8, 10 or 12 ft. Round up, never down. A post that is too long gets cut at the top; a post that is too short cannot be fixed.

Hole diameter is about 3 times the post width. Anything narrower leaves too little concrete around the post to hold it.

Depth by fence height

Fence height	One-third rule embed	Gate and corner posts	Common practice in mild, firm soil
3 ft	24 in (minimum)	30 in	24 in
4 ft	24 in	30 in	24 in
5 ft	30 in	36 in	24 to 30 in
6 ft	36 in	42 in	24 to 36 in
8 ft	48 in	54 in	36 to 48 in

If your frost depth plus 6 in is deeper than the number in this table, the frost figure wins.

Frost depth by region

Frost depth varies by county, not just by state, and your building department sets the number you have to use. These are rough regional ranges to show why the answer changes so much across the country.

Region	Typical frost depth	Likely embed for a 6 ft fence
Florida, Gulf Coast, southern Texas, southern California	0 to 6 in	set by height, about 36 in
Southeast, Southwest, Pacific coast	6 to 18 in	set by height, about 36 in
Mid-Atlantic, lower Midwest, Pacific Northwest inland	24 to 36 in	36 to 42 in
Upper Midwest, New England, mountain states	42 to 60 in	48 to 66 in
Northern Minnesota, North Dakota, northern Maine	60 in or more	66 in or more

In the colder rows, frost depth drives the post length, not fence height. A 6 ft fence in a 48 in frost zone needs 54 in in the ground and a post of at least 10 ft 6 in, so you buy 12 ft posts.

Hole diameter

Post	Line post hole	Corner, end and gate post hole
4x4 wood (3½ in actual)	10 in	12 in
4x6 wood	12 in	12 in
6x6 wood (5½ in actual)	12 in, wider in loose soil	12 in, wider in loose soil
5x5 vinyl	10 in	10 in
4x4 vinyl	8 in	8 in
Chain link, 1⅝ in line	6 in	not applicable
Chain link, 2⅜ in terminal	not applicable	8 in, 12 in for gates

Post length

Fence height	Embed depth	Total needed	Stock length to buy
4 ft	24 in	6 ft	8 ft
4 ft	36 in	7 ft	8 ft
6 ft	24 in	8 ft	8 ft
6 ft	36 in	9 ft	10 ft
6 ft	42 in	9 ft 6 in	10 ft
6 ft	54 in	10 ft 6 in	12 ft
8 ft	36 in	11 ft	12 ft
8 ft	48 in	12 ft	12 ft

For vinyl, the post lengths are fixed by the product line (9 ft for 6 ft privacy is typical). If your frost depth needs more embed than that allows, the calculator warns you to order longer posts. For chain link, line posts are fence height plus 2 ft and terminal posts are height plus 3 ft.

Soil changes the answer

The depth rules above assume ordinary, firm soil. Adjust for what you actually dig into.

• Sandy or loose soil holds a post poorly. Go deeper than the table and widen the hole so there is more concrete bearing on the soil.
• Clay holds well when dry but swells when wet and can heave like frost. A gravel base under the footing and a slightly wider hole help it drain.
• Rock may stop you short of full depth. If you hit ledge, a shallower hole drilled into the rock and grouted can hold, but ask a local pro before you rely on it.
• High water table or wet ground means posts sit in water. Use UC4B rated wood posts and a gravel base, and slope the top of the concrete away from the post.

Every hole in this calculator gets a 6 in gravel base, which lets water drain away from the bottom of the post instead of pooling in a concrete cup.

Before you dig

Call 811 a few business days before you dig. Post holes go 2 to 4 ft down, deep enough to reach buried gas, electric and water lines. The service is free.

GuideHow deep should fence posts beFence post depth by fence height, frost line and soil, with hole diameters, gravel base, gate post rules and the post length to buy for each fence.