How to plan a wood fence
Sections = fence length ÷ post spacing (rounded up); posts = sections + 1 for a straight run (add one for every corner, gate and end of a separate run). Rails = sections × rails per section. Pickets = fence length in inches ÷ (picket width + gap).
Example: 100 ft with 8 ft spacing: 13 sections, 14 posts, 39 rails (3 per section), and 100 × 12 ÷ 5.5 = 218.2 → 219 pickets for a privacy fence.
Rails: 2 or 3?
Fences up to about 4 ft usually use 2 rails; 6 ft privacy fences typically use 3 to reduce warping.
Setting posts
A common rule is to bury one-third of the post's above-ground height, and below the frost line in cold climates. Call 811 before you dig, and check HOA rules and local height limits.