The run decides almost everything
A wood fence is planned from one measurement: the total run. Everything else follows by division. Posts stand 6 or 8 ft on center, so the run divided by the spacing, rounded up, is the section count, and posts are sections plus one. Eight feet is the standard bay for a reason that has nothing to do with the posts — rails sell in 8- and 16-ft stock, so an 8-ft bay frames itself without cutting. The fence materials calculator turns the same run into the full list — posts, rails, pickets, the post length to buy, hole dimensions, and concrete — and charts the concrete cost at both spacings.
This guide’s running example is the classic quote: 100 ft of 6-ft-tall privacy fence, posts 8 ft on center, no gates. Sections: ceil(100 ÷ 8) = 13, so 14 posts. Tightening to 6-ft spacing makes a noticeably stiffer fence — worth weighing for tall privacy runs on windy sites or where the grade steps often — but it costs roughly a third more posts, holes, and concrete. On a 200-ft run the difference is concrete-truck real: 26 posts at 8-ft spacing against 35 at 6-ft.
Rails and pickets follow the style
Rails are counted per section, and the count follows the height. Two rails carry a fence up to about 5 ft tall; at 6 ft and above, a third rail keeps the pickets from cupping and the panel from bowing between posts. A 6-ft privacy fence therefore takes 3 rails per section — 13 × 3 = 39 rails for the example — while a classic picket fence under 4 ft takes 2, and post-and-rail is simply a 2- or 3-rail look chosen by eye and stock length.
Pickets are a pitch calculation: the run in inches divided by the covered width of one picket, meaning its face plus the gap to the next one. A standard 5-1/2 in dog-ear picket butted tight covers 5.5 in — about 2.2 pickets per foot — so the 100-ft privacy run needs ceil(1,200 ÷ 5.5) = 219 pickets. Space the same pickets 1-1/2 in apart for a classic picket look and each covers 7 in; the identical run drops to 172. One caution that saves an argument next season: pressure-treated pickets usually arrive wet and lose width as they season, so a fence butted tight in spring will open small gaps by fall.
A third of the post is underground
The trade rule of thumb buries one-third of the fence height, with a 2-ft minimum. A 6-ft fence buries max(2, 6 ÷ 3) = 2 ft, making the post 6 + 2 = 8 ft — conveniently an even stock length, since posts sell in 8-, 10-, and 12-ft lengths. An 8-ft fence buries about 2 ft 8 in and needs 10.67 ft of post, which rounds up to a 12-ft buy with a cutoff. Two things outrank the rule: in cold climates the local frost depth often governs embedment — a post that stops above the frost line can heave — so check the requirement with your building department; and before any digging, call 811 for utility locates.
The hole is wider than the post and deeper than the bury. Practice is a hole three times the post width — 10.5 in across for a nominal 4×4, whose actual width is 3.5 in — dug 4 in deeper than the bury for a gravel drainage base under the post. The example fence’s holes are 10.5 in across and 2 × 12 + 4 = 28 in deep, with each post standing on the gravel and 24 in of concrete wrapped around it.
Buy the concrete once, on the total pour
The concrete around one post is the hole’s cylinder minus the post itself, over the bury depth. For a 4×4 in its 10.5 in hole that nets about 1.03 ft³ per hole, and the example’s 14 posts add up to 14.46 ft³. Premix yields are 0.60 ft³ per 80-lb bag and 0.45 ft³ per 60-lb bag, so the job takes 25 bags of 80-lb premix, or 33 of 60-lb — rounded up once, on the total.
That “once” is worth money. The per-hole figure tempts a two-bags-a-hole habit, and rounding up at every one of 14 holes buys 28 bags where the pooled total needs 25. The gap widens with the run: the 150-ft version of the same fence sets 20 posts, pours about 20.6 ft³, and needs 35 bags bought as a single quantity — several fewer than hole-by-hole rounding would stack on the cart. Concrete doesn’t care which hole it came from; buy it like one pour.
Gates, and the list end to end
Gates ride on top of the line math. Each gate adds one post — the opening is framed by two posts, one of which would otherwise be shared with the neighboring bay — plus a hardware set of hinges and a latch, while the gate leaf itself is usually built from picket and rail stock already in the count. For gates wider than about 4 ft, upgrade the hinge post to a 6×6 or a steel post and set it in a deeper, wider footing than the line posts; a wide leaf works its hinge post like a lever every time it swings.
The 100-ft privacy fence, end to end: 13 sections and 14 posts at 8 ft on center; 39 rails; 219 pickets butted tight; 8-ft posts from the 2-ft bury; holes 10.5 in across and 28 in deep over the 4-in gravel base; and 14.46 ft³ of concrete bought as 25 × 80-lb bags. Every line traces back to the run, the height, and the style — which makes a fence one of the few projects you can genuinely check a contractor’s material quote on from the curb, before a single hole is dug.