How Deep Should Fence Post Holes Be?

QUIKRETE’s project guide for setting posts says the hole depth should be one-third of the overall post length, plus 6 inches for a gravel base. For a post used for structural support, the same guide says the hole must go at least 6 inches below the frost line. In the one city memo checked for this guide, from Columbus, Ohio, the frost line depth is 32 inches.

QUIKRETE’s rule: one-third of the post in the ground

QUIKRETE’s rule is stated in terms of the whole post, not the part you see. The hole depth should be 1/3 the overall post length, plus 6 inches for the gravel base.

Worked through for a 9-foot post: one-third of 9 feet is 3 feet in the ground, which leaves 6 feet above it. Add the 6 inches of gravel under the post and the hole is 3 feet 6 inches deep, or 42 inches.

Put the other way around, one-third of the post in the ground is the same as half the above-ground height below ground: 3 feet below ground for 6 feet above.

Post length	In the ground (1/3)	Above ground	Hole depth with 6 in of gravel
6 ft	2 ft	4 ft	30 in
8 ft	2 ft 8 in	5 ft 4 in	38 in
9 ft	3 ft	6 ft	42 in

The table is this site’s arithmetic on QUIKRETE’s rule. The guide itself gives the rule, not these rows.

How wide to dig, and what goes in the bottom

The same guide says the diameter of the posthole should be 3 times the post diameter. For a post 4 inches across, that is a 12-inch hole.

Before the post goes in, it says to tamp the sides and bottom of the hole until firm and place 6 inches of gravel or crushed stone in the hole to aid in drainage. For posts set in loose or sandy soil, it says to dig larger, dish-shaped holes.

The concrete does not fill the whole hole. QUIKRETE’s steps have you pour the mix until it reaches 3 to 4 inches from the top, then fill the remainder of the hole with soil dug from it.

That matters when you work out how much to buy: on the Post holes tab of this site’s Concrete Calculator, put the depth of the concrete in the “Hole depth” box, not the full depth of the hole. In a 42-inch hole with 6 inches of gravel at the bottom and the mix stopped 3 to 4 inches from the top, the concrete is 32 to 33 inches deep.

With this site’s defaults of 10% extra and 0.60 cu ft per 80 lb bag, four holes 12 inches wide with 32 inches of concrete take 16 bags. The calculator does not subtract the post, so the count runs a little high.

Frost depth

QUIKRETE’s guide brings in frost for one case. When the post is to be used for structural support, such as for decking, it says the hole must extend at least 6 inches below the frost line. The guide does not state that requirement for fence posts.

QUIKRETE’s guide does not say how deep the frost line is. Here is one city’s figure. A memo from the City of Columbus, Ohio, to its construction industry lists the city’s design criteria under the Residential Code of Ohio, and gives a frost line depth of 32 inches. A note to that table says the jurisdiction fills in the frost line depth column with the minimum depth of footing below finish grade.

That is one city’s number, shown as an example of where to look. The Columbus memo lists climatic and geographic design criteria under the Residential Code of Ohio, and it does not mention fences.

Put next to the rule of thumb, Columbus’s 32 inches is deeper than the 24 inches a 6-foot post gets from the one-third rule, and shallower than the 36 inches a 9-foot post gets.

Permits and local rules

Some cities publish a minimum depth in their fence handouts, and the numbers are not the same from one city to the next. Three examples:

City	What its document says
Sycamore, Illinois	Fence post holes shall be a minimum of 24 inches deep
Momence, Illinois	Post holes must be 36 to 42 inches deep to support the fence
Columbus, Ohio	Frost line depth of 32 inches; the memo does not mention fences

Both Illinois handouts are about more than depth. Sycamore’s says it is for informational purposes only and that the provisions of the city’s adopted codes prevail. Momence’s is dated July 2012. Sycamore’s says a permit is required and must be obtained before a residential fence is built. Momence’s says the city building inspector makes two site visits for a fence, one of them to inspect the location and depth of the post holes.

Set against QUIKRETE’s rule, a 9-foot post has 36 inches in the ground and a 42-inch hole. That 42-inch hole clears Sycamore’s 24-inch minimum and is at the top of Momence’s 36-to-42-inch range. A 6-foot post has 24 inches in the ground and a 30-inch hole, which would be short of Momence’s 36 inches.

These are three cities, not a national rule. Before you dig, ask your local building department three things: whether the fence needs a permit, how deep the post holes have to be, and whether someone has to inspect the holes before you fill them.