About Fencing Calculations
Fence math comes down to spacing. Posts usually sit 6 to 8 feet apart on center, and that spacing sets everything else. Divide your total run by the spacing, add one for the end post, and you have your post count. Rails and pickets follow from there.
Corners and gates throw off a clean count, so handle them separately. Every corner needs its own post, and a gate adds two posts plus hardware. Map the layout before you order, because a fence that's 'about 100 feet' on a tape might hide three corners you didn't budget for.
Pickets are a coverage problem. A 6-inch picket with no gap covers differently than the same picket spaced for a shadowbox, so decide your style first. For chain link, you're buying linear feet of fabric plus terminal and line posts. And don't forget concrete: each post wants a hole about three times the post's width, filled with concrete, which adds up on a long run.
Pros run these numbers to avoid the half-finished-fence problem. Come up two posts short and the job stalls while you wait on a supply run. Figure posts, rails, pickets, and concrete in one pass, add a little overage on pickets for off-cuts, and dig once.