How this instrument works
A board-on-board fence is a privacy-fence style built from vertical boards attached to both faces of the rails in an alternating, overlapping pattern, so each board covers the gap of the one behind it and no sightline gaps show through, even as the wood shrinks and swells with the weather. Because boards overlap, each one doesn't advance the fence by its full width — it only adds its width minus the overlap, since the overlapped portion is already covered by the neighboring piece.
That exposed width — board width minus overlap — is the number that actually matters for counting boards, not the board's full width alone. A 6 in wide piece overlapped 1 in with its neighbor only advances the fence line by 5 in of new coverage; the other 1 in duplicates ground the previous board already covered. Dividing the fence's total length by that exposed width, rather than by the raw board width, gives the accurate count of boards needed.
This instrument rounds the raw division up using a ceiling function, since a fence that doesn't divide evenly into whole exposed-width increments still needs one more full board to close out the run — a partial piece isn't buildable, so the leftover distance gets covered by a full board with its overlap pulled slightly tighter, not left as a gap.
Overlap width is a design and privacy choice: a wider overlap gives more assurance against sightlines and shrinkage gaps opening up over time but uses more boards, and more material, per foot of fence, while a narrower overlap covers the same length with fewer boards but leaves less margin if the boards shrink.
- Enter the total fence run into Fence length (ft) — the straight-line distance the fence needs to cover.
- Enter the board's full width into Board width (in) — the actual width of the board stock being used, not the overlap alone.
- Enter the planned overlap into Overlap between boards (in) — how much each board covers of its neighbor.
- Read Boards needed beneath the inputs — a whole-number count already rounded up to close the run.
- Board width must exceed the overlap; a board that overlaps its own full width would never advance the fence line at all.
Worked example — a 50 ft fence, 6 in boards, 1 in overlap
Enter 50 into Fence length (ft), 6 into Board width (in), and 1 into Overlap between boards (in). Each board's exposed width is 6 − 1 = 5 in. The instrument converts 50 ft to 600 in and divides by that 5 in exposed width: 600 / 5 = 120 exactly. Boards needed reads 120.
Those 120 boards, each 6 in wide but effectively advancing the fence 5 in at a time, cover the full 600 in (50 ft) run with every seam behind them overlapped by 1 in — enough for the fence to present a solid face from either side even as individual boards shrink slightly with the seasons.
Questions
Why divide by board width minus overlap, not board width alone?
Because overlap means part of every piece duplicates ground the previous piece already covered, so only the non-overlapped portion — board width minus overlap — actually extends the fence line. Dividing fence length by the full board width would undercount, since it would ignore that each piece only contributes its exposed width toward covering new ground.
What overlap should I use for a board-on-board fence?
It's a design and privacy trade-off rather than a fixed rule — a wider overlap gives more margin against sightlines and shrinkage gaps opening up over time, at the cost of needing more boards, and more material, to cover the same fence length. A narrower overlap stretches the same board count further but leaves less buffer if the boards shrink or warp.
What if overlap is equal to or larger than the board width?
The instrument won't compute a valid count, because board width minus overlap would be zero or negative — a board that overlaps its entire width, or more, never actually advances the fence line, so there's no valid exposed width to divide the fence length by. Overlap must be smaller than the board width.
Why does the result round up instead of down?
Because a fence length that doesn't divide evenly into whole exposed-width increments still needs a complete extra board to close out the run — there's no such thing as a fractional board covering the leftover distance. Rounding up means the very last board's overlap may sit slightly tighter than the target, not that a gap gets left uncovered.
Is a board-on-board fence the same as a shadowbox fence?
No, though they're often confused — a board-on-board fence overlaps boards on alternating faces of the same rail line so no light gaps show through from either side, while a shadowbox fence alternates boards on opposite sides of the rail with a small gap between them, letting some light and airflow through while still blocking a direct sightline. This calculator's overlap-based formula applies specifically to that style, where boards genuinely overlap rather than merely alternate.
Does this account for end posts or corner boards?
No — it counts the field boards needed to cover a straight run of the given length using the specified width and overlap. Corner boards, gate openings, and any end-post trim details fall outside this uniform-spacing count and should be planned and ordered separately from the field-board total this instrument returns.