Fence Post Spacing and Concrete: How to Get It Right

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Recommended Maximum On-Center by Fence Type and Height

A fence post is a cantilever in dirt. The infill is the sail. Tighter on-center is how you share wind and keep panels from sagging. These are the maximums this site uses as a starting point — not an engineered wind spec, and not IRC deck-beam spacing.

Privacy is the heaviest sail. Field fence is mostly air. If your panel module is wider than the recommended max, cut the module or add a post — do not stretch the last bay and hope. The calculator caps a too-wide preference and warns.

Wind Exposure Drops the Maximum

Sheltered means buildings or a tree line on the windward side. Open is a typical yard with a fetch. High is a ridge, coast, or open field. The rule this calculator uses:

A 6 ft privacy fence that is 8 ft OC in a sheltered side yard becomes 7 ft OC in the open and 6 ft OC on a windy lot. It never goes below 4 ft on this sheet. If the inspector or the fence maker publishes a tighter number, use theirs.

Working spacing is the preferred module if you entered one and it is at or under the recommended max. If the field is blank (or not a positive number), working spacing is the recommended max. Then:

Actual OC is usually a little tighter than the working number because ceil adds the extra post that closes the last bay. That is what you snap on the string.

Hole Diameter: Three Times the Post, Even Inches, Minimum 10"

A hole that is barely bigger than the post does not give the backfill a collar. The field rule is about three times the post width, then rounded up to the next even inch, never under 10".

Use actual size, not the nominal 4" on a 4×4 ticket. You can dig larger than the suggestion (rocky soil, a sloppy auger). Digging smaller than about 3× is how posts lean in the first season. See leaning posts.

Hole Depth: 24" Floor, Frost Line, or One-Third of Height

The old carpenter line is “bury one-third of the post.” That is a height rule, not a frost rule. Ice under a shallow hole lifts the post and the panel with it. This site takes the maximum of three numbers:

A 6 ft fence in moderate frost: max(24, 36, 24) = 36". The same fence with no frost: max(24, 18, 24) = 24". An 8 ft privacy fence in mild frost: max(24, 24, 32) = 32". Your adopted frost map may be deeper than this climate picker. If the two disagree, dig to the map.

Concrete Volume: Cylinder Minus the Post, Then Divide by 1728

The hole is a cylinder. The post occupies some of that cylinder. You pay for the difference, and only for the depth that is actually concrete (hole depth minus gravel).

Areas are in square inches and depth is in inches, so the product is cubic inches. 1728 cu in = 1 cu ft. If someone types a hole smaller than the post, volume goes to zero instead of going negative. Gravel volume on this sheet is the full cylinder of gravel thickness — we do not subtract the post from the gravel bed, because that bed is placed before the post is fully in and the numbers stay conservative.

Ready-mix for a long run is often cheaper once you clear about a yard. Convert with total cu ft ÷ 27. For mixed shapes (a pad plus posts), use the concrete volume calculator and add this post takeoff. Waste and bag-vs-truck: concrete volume guide.

Bag Yields: 0.45 cu ft (60 lb) and 0.60 cu ft (80 lb)

Those are the yields this calculator uses. They match common bag labels for pre-mix, not a lab density. Always round up:

Buy a couple of extra 60s. An auger that walks, a hole that bells out, and a mix that takes more water than the bag wanted will eat the theoretical count. Do not water bags into soup to stretch volume — you lose strength at the grade line, which is exactly where posts rot and lean.

Concrete is not mandatory on every fence. In free-draining soil with no frost, packed gravel often holds a line post and lets water leave. In wet clay, a sealed concrete cup can hold water against the post. Walk that choice in concrete vs gravel vs packed dirt before you order 40 bags.