How this calculator works
One face carries boards spaced apart, and the other face carries boards offset by half that spacing, so every gap is covered from behind. If a board is 140 mm wide and the layers overlap 20 mm at each edge, the boards on one face sit 240 mm apart and the far layer fills the remaining 120 mm. This is why a board-on-board fence uses about twice the board of a single closed face. Posts and rails follow the usual rule: one more post than sections, and a set number of rails per section.
Formula
Pitch = 2 x (board width - overlap); boards per face = ceil(run / pitch); boards = 2 x boards per face. Sections = ceil(run / post spacing); posts = sections + 1; rails = sections x rails per section. Board length = boards x fence height.
Worked example
A 30 m run with 140 mm boards and 20 mm overlap has a 240 mm pitch, so each face needs 125 boards and the fence needs 250. At 1.8 m high that is 450 m of board. With 2.4 m post spacing it has 13 sections, 14 posts and 26 rail boards.
Advertisement
Frequently asked questions
Why does board-on-board need about twice the boards?
Each side is a full layer of boards. The second layer hides the gaps of the first, so the two faces together cover the run twice over. A single closed face uses roughly half the board.
What overlap should I use?
Enough that seasonal shrinkage cannot open a gap: 20 to 25 mm on each edge is typical for 140 mm boards. Check the boards are straight, because the second layer follows the first.
Does this count concrete for the posts?
No. Post holes and bagged concrete are estimated separately; use the fence or post hole concrete page once you have the post count.