Fence post depth and frost line

The deeper of two numbers: a third of the post length, or the frost line plus 6 inches. Sorted by frost depth, so the states where the second rule takes over are at the top.

Two rules, and the deeper one wins

The one-third rule is about leverage. A fence is a lever with wind pushing on it, and burying a third of the post gives the soil enough grip to resist. It scales with fence height, which is why the columns above differ in warm states.

The frost line is about heave. Water in the soil freezes, expands and lifts whatever sits above it — unevenly, so a fence does not rise, it leans. It does not scale with fence height at all, which is why the columns are identical in cold states: at that point the frost line is the only thing setting the depth.

Reading the last column

It is the part people forget when they order. A six-foot fence in Florida takes an eight-foot post; the same fence in Minnesota takes a twelve — because the post has to be long enough to bury 66 inches of it. That changes the price of every post and it is not something you can discover after the holes are dug.

These are a starting point, not an authority

Frost depth is adopted locally and varies inside states with elevation, soil and how conservative the jurisdiction is. The figures here are compiled design depths, given at the deeper end where a state spans a range — running a post short is not a mistake you can correct later. Your building department’s number is the one that counts, and it costs one phone call.

Using it

The fence calculator applies this automatically from your state, tells you which of the two rules is driving the depth, and works out the posts, rails, pickets and bags of concrete that follow from it.