How To Calculate Fence Posts

The post count comes from a simple rule: divide the fence length by the spacing and add one. The extra post closes the far end, and forgetting it is the single most common ordering mistake.

Spacing is measured centre to centre, not the gap between the posts. Eight feet on centre means the boards between two posts span slightly less than eight feet because the posts themselves take up width.

Sections are the spaces between posts, and there is always one fewer section than there are posts. A 120 foot run at 8 foot spacing gives 15 sections and therefore 16 posts, which is the classic off-by-one that catches people out.

Rails are the horizontal members that run between posts. Two rails suit a fence under four feet, three is the standard for a six foot privacy fence, and four is used for tall or exposed sites.

Rails are counted per section, not per post. Fifteen sections with two rails each need 30 rails, and buying one per post leaves the fence short by fifteen pieces.

Pickets are counted from the width plus the gap. A 3.5 inch picket with a 2 inch gap occupies 5.5 inches of run, so 1440 inches of fence holds about 262 pickets, plus one to close the last gap.

The gap choice changes the count sharply. Narrowing the gap to 1.5 inches drops the pitch to 5 inches and raises the picket count to 289 for the same 120 feet, so decide the gap before ordering.

Corner and end posts are heavier than line posts. They carry the pull of two runs and are usually set in deeper concrete, so order 4x6 posts for corners and 4x4 for the line.

Concrete is counted per post. Two 60 pound bags per post is the usual allowance for a six foot fence in decent soil, and three or four for corners, gates and sandy ground.

Gates need their own hardware. A gate opening is normally four feet, which removes about one section of pickets and adds a hinge post and a latch post, so plan the opening before counting.

Slopes change the maths. On a hillside the panels are stepped or racked, which changes both the rail lengths and the picket count, and posts may need to be longer at the low end.

Always buy a few spares. Posts warp, pickets split and rails crack, and a small surplus avoids a second trip for a handful of pieces.

Line posts are set every section, but corner and gate posts are heavier, and a fence built entirely from line posts will lean at the first corner within a season.

Pickets are sold in bundles that rarely match the count exactly, so round up to the next full bundle and keep the remainder for repairs.

A shadowbox fence alternates pickets on one side or the other, which hides the gaps from both directions but uses about the same number of boards as a solid privacy fence.

Post concrete should be mixed stiff and crowned at the top so water runs away from the post instead of pooling against the cut end.

Local codes often limit front garden fence height and require the finished face to sit on your side of the boundary, so check before ordering.