Board on Board Fence Calculator
Calculate posts, rails and overlapping boards for a board-on-board privacy fence, plus material cost in USD.
Board-on-Board Fence Calculator: Posts, Rails and Overlapping Boards
A board-on-board fence (sometimes called a shadowbox or good-neighbour fence) is a privacy-fence style where boards overlap on alternating sides of the rail, so there's no straight sightline through the fence from either side — unlike a standard picket fence, where a single row of boards leaves a gap unless it's built with zero spacing. That overlap means the boards' effective spacing is narrower than the board width itself, so this calculator uses a different formula from a plain picket fence: it works out posts, rails and boards from your fence length, post spacing, board width and overlap, then adds a waste allowance and an estimated material cost.
How to use the board-on-board fence calculator
- Enter the total fence length you're building.
- Set the post spacing (on-centre) and the fence height.
- Enter the board width and the overlap between adjacent boards — a common starting point is about 24% of the board width.
- Choose the number of rails per section (3 is standard for a 1.8m privacy fence).
- Set a waste allowance, then read off the total posts, rails, boards and estimated cost.
The formula, with a worked example
Posts follow the standard fencepost rule: posts = ceil(fence length ÷ post spacing) + 1. Boards are different: because each board after the first only advances the run by its width minus the overlap, the effective pitch is (board width − overlap), and boards = ceil((fence length − board width) ÷ effective pitch + 1), then increased for waste.
Worked example: a 30m run, posts every 2.5m, 3 rails per section, 14cm-wide boards with a 3.36cm overlap (24%), and 10% waste. Posts = ceil(30 ÷ 2.5) + 1 = 13, giving 12 sections and 36 rails. Effective pitch = 14 − 3.36 = 10.64cm, so raw boards = (3000 − 14) ÷ 10.64 + 1 ≈ 282. Adding 10% waste and rounding up gives 310 boards — noticeably more than a same-width picket fence with no overlap would need, because every board is doing double duty covering the gap on the opposite face.
Tips and common mistakes
At a typical US price of around $18 per post, the worked example above (13 posts for a 30m run) would come to roughly $234 in posts alone, before rails and boards are added — use the calculator's cost output for the full estimate in USD.
In the US this style is commonly called a "shadowbox fence" or "good neighbor fence" — boards alternate sides of the rail so the fence looks finished and fully private from both yards, unlike a single-sided stockade fence.
- Budget for more material than a standard fence. The overlap means every metre of run uses more linear board footage than a plain picket or lapped fence — don't reuse a standard picket-fence estimate for a board-on-board job.
- A wider overlap looks better but costs more. A generous overlap (25-30% of board width) hides shrinkage gaps as boards dry out, but pushes the board count up — the calculator lets you compare overlap percentages before you order.
