Fence Calculator
Posts, rails, pickets and concrete — with the extra post.
Work out Fence. Posts, rails, pickets and concrete — with the extra post. Free, with no account and nothing to download.
Zero for a solid privacy fence.
Often a third of the above-ground height, and below the frost line.
A nominal 4×4 measures 3½.
Posts needed
14
13 sections at 7.69 ft actual spacing
13 sections need 14 posts, because both ends need one — the classic off-by-one, and it always errs short. Note the spacing has become 7.69 ft rather than the 8.0 ft asked for: the run rarely divides evenly, and equal sections beat a short one at the end. The concrete figure subtracts the post from the hole, which a 4×4 in a 10-inch hole reduces by about a sixth.
How the Fence Calculator works
The fencepost problem, and it is called that for a reason: a run divided into five sections needs six posts, because both ends need one. That off-by-one is the commonest error in fence estimating and it always errs short. This also subtracts the post from the hole when working out concrete, which matters more than people expect — a 4×4 in a ten-inch hole displaces about a sixth of the mix.
Also known as: how many fence posts do I need · fence picket calculator · fence material calculator · privacy fence calculator
The fencepost problem
A run divided into five sections needs six posts, because both ends need one. It is the canonical off-by-one, it has a name for a reason, and it always errs short.
The second thing the arithmetic reveals is that your chosen spacing is rarely the spacing you get. A hundred feet at eight-foot spacing is 12.5 sections, which is not a thing — so it becomes thirteen sections at 7.69 feet each.
Equal sections beat a full-length run with a short one at the end, both structurally and visually, which is why the count is rounded first and the spacing derived from it.
In practice
A hundred-foot run at eight-foot spacing: thirteen sections, fourteen posts, actual spacing 7.69 feet. Two rails a section is 26 rails.
Pickets go by pitch rather than by section. A 5½ inch picket with a 2 inch gap has a 7½ inch pitch, so 1,200 inches of run takes 160 pickets.
Concrete is where the post itself matters. A ten-inch hole two feet deep is 1.09 cubic feet, and a 4×4 — actually 3½ inches — displaces 0.17 of that. So 0.92 per post, and 12.89 cubic feet across fourteen posts. Ignoring the post over-orders by about a sixth.
Where the figure deceives
It assumes a straight run. Corners and gates each need their own posts, and a gate post usually needs to be larger and set deeper than a line post — so a fence with three gates needs more than this shows.
Post depth is a local question. The common guide of a third of the above-ground height is fine until frost is involved, and below-frost-line depth is regional. A post set above the frost line heaves.
Terrain matters too. A run measured flat across sloping ground is shorter than the fence you will actually build, and stepped fences need extra picket length at each step.
Acting on it
Count corners and gates separately and add those posts to this figure.
Check local frost depth before setting post depth, rather than using a rule of thumb.
Measure along the ground on a slope, not across it.
Frequently asked questions
How many fence posts do I need?
Divide the run by your post spacing and round up for the number of sections, then add one. A hundred feet at eight-foot spacing is thirteen sections and fourteen posts — and the actual spacing becomes 7.7 feet rather than 8.
How far apart should fence posts be?
Six to eight feet is usual for timber, with closer spacing for tall or solid panels that catch more wind. The run rarely divides evenly, so the real spacing is the run divided by whole sections.
How many pickets do I need?
Divide the run in inches by the picket width plus the gap. A 5½ inch picket with a 2 inch gap has a 7½ inch pitch, so a hundred-foot run takes 160.
How much concrete per post?
The hole volume less the buried part of the post. A ten-inch hole two feet deep is about 1.09 cubic feet, and a 4×4 displaces around 0.17 of that — so about 0.92 per post, which this accounts for.
How deep should the posts go?
A common guide is a third of the post's above-ground height, and below the local frost line where that applies. Frost depth is regional, so check locally rather than following a general figure.
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