Fence post spacing and how the material counts add up
How post spacing sets your section count, why posts are sections plus one plus gates, and how picket width drives the board total.
Post spacing sets the section count
Everything on a fence starts with how the run is divided. The tool takes your total length, divides it by the post spacing, and rounds up, since a partial bay at the end still needs its own section. A 100 foot run at 8 foot spacing gives 12.5, which rounds up to 13 sections. Tighter spacing means more sections and a stiffer fence, while wider spacing uses fewer posts but asks more of each rail. Six to eight feet is the usual range for wood fences.
Why the post total is sections plus one plus gates
Picture a straight fence: every section is bounded by a post, and the sections share posts in the middle, so a run of N sections is capped by N plus 1 posts from end to end. Gates break that pattern because each one needs its own post on the swinging side, so the tool adds one post per gate on top. That is how a 13 section fence with a single gate lands at 15 posts. If the count looks high, check whether you meant to include the gate.
Rails and pickets scale differently
Rails follow the sections: multiply the section count by the rails per section, so 13 sections at 3 rails each is 39 rails. Pickets ignore the posts entirely and follow the run length instead. The tool turns the length into inches and divides by the picket face width, rounding up, on the assumption that boards sit edge to edge. A nominal 1x6 dog ear picket has about a 5.5 inch face, which is the default. Change the face width and the picket total moves while sections, posts and rails stay put.
Adjusting for real world layouts
The estimate models one straight run with tightly butted pickets, which is a clean starting point rather than a full takeoff. If your fence turns corners, add a post for each turn and treat each leg as its own run. For a spaced picket or shadowbox design you will use fewer boards than a solid privacy wall, so enter a wider effective picket width to approximate the reveal. Buy a few extra pickets and a spare post to cover cuts, splits and the odd bad board.