How deep should a fence post be?
The calculator takes the deepest of three answers: a share of the fence height, a fixed minimum, and your frost line. Whichever is largest sets the hole, and the result tells you which one it was.
Hole depth = the larger of (height above ground ÷ 2 + 6 in gravel), 24 in, and (frost depth + 6 in)
Corner and gate posts get 12 in more on the height rule, and sandy or wet soil gets 6 in more.
Where the rules come from
The height rule. Quikrete's "Setting Posts" project sheet says the hole should be one third of the post's overall length, plus 6 in for a gravel base. A post with one third of its length in the ground has half as much in the ground as above it, so a 6 ft fence on a 9 ft post has 3 ft buried. Quikrete's online post calculator uses the same half-of-height figure. Its setting-posts web page gives a looser range of one third to one half of the above-ground height, which for a 6 ft fence starts at 2 ft, so treat the calculator's number as the sturdy end of that range rather than the least you can get away with.
Line posts versus corner and gate posts. Penn State Extension, writing about horse fences, sets line posts 36 in deep and says corner and gate posts are usually about 25 percent thicker and often 48 in deep. They carry the pull of the whole run or the weight of a swinging gate. That 12 in difference is the extra the calculator adds when you pick "Corner or gate". Penn State also says sandy or wet soil needs deeper posts, but gives no figure; the 6 in the calculator adds is our allowance, not theirs.
The minimum. For chain link, the Chain Link Fence Manufacturers Institute cites ASTM F567: footings at least 24 in deep, plus 3 in for every foot of fence height over 4 ft. The calculator uses 24 in as the floor for every post type, and for round metal posts it also shows the ASTM figure for your height so you can check against it.
The frost line. Quikrete says structural posts need the hole at least 6 in below the frost line, and the International Residential Code puts footings "below the frost line" without giving a number. Local codes set it. Minnesota's rule 1303.1600 is a good example of how much it varies inside one state: 42 in in Zone II and 60 in in Zone I. If you enter a frost depth, the calculator adds 6 in and uses that when it is the deepest of the three.
A worked example
A 6 ft privacy fence on 4x4 line posts in ordinary soil, with no frost depth entered:
- Height rule: 72 in ÷ 2 = 36 in in the ground, plus 6 in of gravel = 42 in (3 ft 6 in).
- The 24 in minimum is smaller, so the height rule wins.
- Hole width: three times the 3.5 in post = 10.5 in.
- Post length: 6 ft above ground plus 36 in in concrete = 9 ft, so buy 10 ft posts.
- Concrete: the 36 in of hole around the post holds 1.55 cu ft, which is 4.1 bags of 50 lb fast-setting mix per post.
Move the same fence to Minnesota's Zone II and enter 42 in of frost: the frost rule now gives 48 in, which beats 42 in, so the hole goes to 48 in and each post takes 4.8 bags. In Zone I at 60 in, the hole is 66 in and you need 12 ft posts.
Recommended depth by fence height
Every row below comes from the calculator: 4x4 wood posts, ordinary soil, no frost depth, 6 in of gravel, 10.5 in holes and 50 lb fast-setting concrete.
| Fence height | Line post hole | Corner or gate hole | Line post to buy | Bags per line post | ASTM chain link footing |
|---|---|---|---|---|---|
| 3 ft | 24 in | 36 in | 6 ft | 2.1 | 24 in |
| 4 ft | 30 in | 42 in | 6 ft | 2.8 | 24 in |
| 5 ft | 36 in | 48 in | 8 ft | 3.4 | 27 in |
| 6 ft | 42 in | 54 in | 10 ft | 4.1 | 30 in |
| 7 ft | 48 in | 60 in | 12 ft | 4.8 | 33 in |
| 8 ft | 54 in | 66 in | 12 ft | 5.5 | 36 in |
The chain link column is the ASTM F567 footing depth, shown for comparison. It is a separate standard written for metal chain link posts.
How wide should the hole be?
Quikrete and Sakrete both say three times the width of the post, so about 10 in for a 4x4 and 16 in for a 6x6. For 5x5 vinyl posts the calculator uses the 12 in hole Weatherables gives. It never goes under 6 in, the width Master Halco gives for chain link line post holes.
Do I need the gravel?
Quikrete's instructions put 6 in of gravel in the bottom of the hole and stand the post on it; Sakrete gives 4 to 6 in, compacted. The calculator counts those 6 in as part of the hole but not as part of the post, which is why a 42 in hole only needs 36 in of post below ground.
What to work out next
Once you know the hole, the fence post concrete calculator turns it into bags for the whole job, and the fence post calculator counts how many posts your fence line needs. For the full list of posts, rails, boards and concrete, use the fence calculator.