How the fence calculator counts posts
The run is divided by the post spacing and rounded up, because a part section still needs a post at each end. A 100 ft run at 8 ft spacing is 12.5, so it becomes 13 sections. A straight run needs one more post than it has sections, which gives 14 posts, and the rails are the sections multiplied by the rails per section: 13 × 3 = 39. Pickets are counted separately. The fence length in feet is multiplied by 12 to get inches and divided by the picket width plus the gap; 100 ft is 1,200 in, and 1,200 ÷ 6 = 200 pickets.
Rails, pickets and the gap
Two rails are common on fences up to about 4 ft high, and taller fences usually get three, which is why the number of rails is an input rather than a fixed figure. The gap between pickets changes the look and the count: a gap of 0 butts the pickets together for a privacy fence, while a small gap lets air and light through. Both the picket width and the gap should be measured in inches, and the last section of the run may be shorter than the spacing without changing the post or picket totals.
One straight run at a time
The calculator assumes a single straight line with a post at each end. Every corner, gate and change of direction you add needs its own post, so work out each straight stretch separately and add the extra posts. To estimate concrete for setting the posts, use the concrete calculator; for the lumber in the rails, the board foot calculator takes the board size and length.
Frequently asked questions
How many posts do I need for 100 ft of fence at 8 ft spacing?
Twelve and a half sections rounds up to 13, and a straight run needs one more post than sections, so 14 posts.
How many pickets for 100 ft with 5.5 in pickets and a 0.5 in gap?
Each picket plus gap covers 6 in, and 100 ft is 1,200 in, so 1,200 ÷ 6 = 200 pickets.
What if the last section is shorter?
That is normal. 100 ft at 8 ft spacing gives 12 full sections (96 ft) and a 4 ft last section. It still counts as 13 sections and 14 posts, and the picket count uses the full length.