Estimate posts, rails and panels needed for a fence run.
Fence material follows from the section count. Divide the run by the post spacing, round up, and you have the number of bays; posts are one more than that because a run has a post at each end. Eight feet is the usual spacing because rails come in 8 and 16 ft lengths, so it wastes nothing. Rails are simply sections times rails per bay — two for a 4 ft fence, three for 6 ft, and a fourth if the boards are heavy or the fence is exposed to wind. Pickets are counted from the pitch, which is the board width plus the gap: a 5.5 in board with a 2 in gap repeats every 7.5 inches, so a 150 ft run needs 240 boards. Allow roughly two bags of postcrete or concrete mix per post.
Fence Material
Sections = run / post spacing, rounded up; posts = sections + 1; rails = sections x rails per section; pickets = run x 12 / (picket width + gap)
A third of the total post length, or at least 2 ft, and below the local frost line. A 6 ft fence therefore wants 8 ft posts set 2 ft deep in concrete.
Because rails and panels are milled in 8 and 16 ft lengths, so 8 ft centres produce no offcuts. Six-foot centres are stronger in high wind but cost about a third more in posts and concrete.