The formula and a worked example
This calculator runs three separate counts off the same fence length. First, pickets — the fence line in inches divided by the width one picket takes up (its own width plus any gap), with your waste allowance added on top:
Pickets = ceil((run length in ÷ (picket width in + gap in)) × (1 + waste %))
Second, posts — the fence length divided by post spacing, rounded up to whole sections, plus one extra post to close off the far end:
Posts = ceil(run length ft ÷ post spacing ft) + 1
Third, rails — one rail board per section, per row, since each gap between two posts is one section that needs its own horizontal support:
Rails = (posts − 1) × rail rows
Worked example: a 100 ft privacy fence built with standard 5.5 in pickets, no gap, 8 ft post spacing, 3 rail rows, and a 5% waste allowance. Pickets: 100 ft × 12 = 1,200 in ÷ 5.5 in = 218.18, × 1.05 waste = 229.09, rounded up to 230 pickets. Posts: 100 ÷ 8 = 12.5, rounded up to 13 sections, + 1 = 14 posts. Rails: 13 sections × 3 rows = 39 rail boards. Those are exactly the numbers the calculator returns for these default values.
Privacy, spaced picket, and shadowbox styles
Privacy fencing uses zero gap — pickets butted edge to edge so there's no line of sight through the fence at all. This uses the most pickets per foot of run, but the least math: just picket width divided into the run length.
Spaced picket fencing leaves a visible gap between boards, commonly 1.5 to 2.5 in, for a more open, decorative look that still defines the property line. Fewer pickets are needed for the same run — the same 100 ft example above, with a 2 in gap instead of a tight fit, drops from 230 pickets down to about 168, since each picket now claims 7.5 in of the run instead of 5.5 in.
Shadowbox fencing (also called board-on-board) alternates pickets on the front and back of the rails, offset from each other, so the fence looks solid from either side while still allowing some airflow through the small gaps. Because pickets are installed on both faces, it needs roughly double the pickets of a single-sided privacy fence — but not quite double, since each side's pickets typically overlap the gaps on the opposite side by an inch or so rather than lining up exactly. As a rough estimate, plan on about 1.7 to 1.8 times the picket count of a standard privacy fence for the same length, then confirm against your specific picket width and overlap once you've settled on a layout.
Why gaps grow (or don't) after installation
Wood pickets are cut from a material that keeps moving after it's nailed up. Green or wet pressure-treated lumber, fresh from the treatment plant, is often still saturated with moisture when it's installed. As it dries out over the following weeks and months, it shrinks slightly in width — which is why many installers set those pickets tight, with no planned gap at all. The gap forms on its own as the wood dries, and building in an additional gap on top of that can leave the finished fence gappier than intended.
Kiln-dried lumber, cedar that's already cured, composite, and vinyl pickets behave differently — they've either already done most of their shrinking or they don't shrink the same way wood does at all. Those materials should be installed with whatever gap you actually want to see in the finished fence, since there's no drying process coming later to open one up for you. Composite and vinyl products specifically should follow the manufacturer's spec sheet, since recommended spacing can vary with the installation temperature and the specific product line.
Don't forget post depth and concrete
This calculator counts pickets, posts, and rails, but it doesn't size the holes those posts sit in. A common starting point is to bury about one third of each post's total length below grade, and to make sure the bottom of the hole sits below your local frost line — in colder climates the frost depth often matters more than the one-third rule, so it's worth checking with your local building department. Once you know your post size, hole diameter, and hole depth, the fence post concrete calculator on this site works out exactly how many bags of concrete you'll need to set each one, so it's worth running your post count from here straight into that tool next.