Fence Ledger guide
The common answer is a rule of thumb. The real answer is a local one, and this is how to find it.
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You will read that a post should be buried about a third of the fence height, with 24 inches as a floor. For a six-foot fence that gives roughly 24 to 30 inches, and it is a reasonable place to start.
It is a rule about leverage. A fence is a lever with wind pushing on the wide face, and the buried portion is what resists it. A third is roughly where that stops being the limiting factor for a typical residential fence.
What the rule does not contain is a climate, a soil type or a building code — and any of those three can override it.
Water in soil expands when it freezes. If the bottom of your post sits above the depth that freezes in your area, the expanding ground pushes it upward — a little each winter, never quite settling back. After a few seasons the posts are proud of the ground, the rails are out of line, and the fix is digging them all out and starting again.
The depth that freezes is the frost line, and it varies enormously. In parts of the southern United States it is effectively zero. In the upper Midwest and northern New England it can exceed four feet. A 30-inch post is generous in one place and a guaranteed failure in the other.
The standard advice is to set the bottom of the post below the local frost line so there is no frozen soil beneath it to lift against.
There are only two sources worth trusting, and neither of them is a website.
Once you have a depth, the post length follows from it, and this is where a lot of material gets wasted or wrongly bought. A post has to be as long as the fence is tall plus everything that goes underground, and the underground part is not only the depth you decided on — most builders put a few inches of gravel under the post for drainage, and the post sits on top of that.
So a six-foot fence with 30 inches of post below grade needs roughly a nine-foot post, and nine-foot posts are not a stock length everywhere. The realistic choices are to buy eight-footers and accept a shallower set, buy ten-footers and cut, or order the odd length. All three are fine; discovering the problem after the holes are dug is not.
The other reason to settle this early is that post length and hole depth together decide how much concrete the job takes, and concrete is usually the item people underbuy. Working the length out at the planning stage means one trip for materials instead of two.
Depth is one half of how a post is held; what fills the hole around it is the other. There are three common approaches and they are not interchangeable, though they are often discussed as if they were.
Concrete is the usual choice in the United States and gives the most rigid result. Its weakness is that a concrete collar can hold water against the post if the hole has no drainage beneath it, which is exactly why the gravel base at the bottom matters — it gives water somewhere to go instead of sitting at the end grain.
Gravel alone, packed in layers, is common in wetter climates and among people who expect to replace or move a fence later. It drains freely and a post can be pulled without breaking up a footing. It gives up some rigidity, so it suits line posts more than gate posts.
Tamped earth is the traditional method and still works in dense, well-drained soil for a low fence. It is the cheapest and the least forgiving of poor soil. Whichever you pick, the corner posts and the gate posts are the ones to over-build: they carry loads the line posts never see, and they are the ones people notice when they sag.
Frost depth is one of a short list of things this calculator deliberately does not preset — along with permit requirements, fence height limits and property setbacks.
The reason is that a plausible-looking number would be worse than no number. A figure on a screen looks checkable. If we printed a frost depth for your state, you would reasonably assume someone had checked it against your county, and nobody had. Better to say plainly that we do not know it and point you at someone who does.
The calculator lets you enter whatever depth your building department gives you, and every quantity that depends on it — concrete especially — recalculates from your figure rather than from ours.
Beyond frost, three things push the number up.
Depth is not just a structural decision, it is a shopping list decision. Concrete volume scales directly with it, and it interacts with hole diameter — which matters more.
Volume grows with the square of the radius, so widening a 10-inch hole to 12 inches raises the concrete needed by about 45%, while going 20% deeper raises it by 20%. If a supplier or a calculator quotes you bags per post without stating both numbers, the figure cannot be checked by anyone.