Fence calculator
A hundred-foot fence at eight-foot spacing is twelve and a half sections, so thirteen sections and fourteen posts — not thirteen. Every run needs one more post than it has sections, because the last section still has to end somewhere. Corners and gates each add another, and a gate post is a heavier section than a line post because it carries a swinging load.
A fence run needs one more post than it has sections. Divide the length by the post spacing, round up for sections, then add one. A 100 ft run at 8 ft spacing is 13 sections and 14 posts, plus an extra post at every corner and gate.
How to count fence materials
Spacing is set by the fence type, not by preference. Panel fencing is fixed at the panel width, usually 1.83 m, and the posts must land exactly there. Rail-and-picket fencing tolerates anything up to about 2.4 m before the rails start to sag. Closeboard with heavy arris rails can stretch to 3 m. Going wider than the timber wants saves posts and costs you a fence that visibly bows within two winters. On a slope, panels either step down in fixed jumps or the fence is built raked with rails following the ground — the two need different post lengths, and stepping wastes the most timber.
Questions
Eight feet or 2.4 m for rail fencing, exactly the panel width for panel fencing, and up to 3 m for heavy closeboard.