What this calculator does
A board-on-board fence overlaps every board with the next, so each one covers less than its own width. With 140 mm boards overlapping 25 mm, the effective spacing is 115 mm and a 20 metre fence takes 174 boards.
The overlap is what makes the fence private, and it is also what makes it expensive. Butting the boards with no overlap needs 143 boards; a 70 mm overlap needs 286, exactly double. Every millimetre of overlap is bought with timber, so the figure is worth settling deliberately rather than by eye.
The formula
Each board advances the fence by its width less the overlap, so the board count is the fence length divided by that effective spacing, plus one to close the run. Posts and rails follow the usual spacing arithmetic. The total lineal length multiplies the board count by the fence height, which is what a timber merchant will quote against.
| Term | Meaning |
|---|---|
| Board-on-board | Overlapping vertical boards giving privacy from both sides with no through-gaps. |
| Overlap | How far each board covers the one before. Larger means more privacy and more timber. |
| Effective spacing | Board width less overlap: the distance each board actually advances the fence. |
| Shadowbox | A related style with boards alternating on each side of the rails, using a similar count. |
The inputs explained
| Field | What to enter |
|---|---|
| Fence length (m) | Fence length in metres. |
| Fence height (m) | Fence height, which sets the lineal board length. |
| Board width (mm) | Board width in millimetres. |
| Overlap (mm) | Overlap between adjacent boards. 20 to 30 mm is typical and allows for timber shrinkage. |
| Post spacing (m) | Post spacing, centre to centre. |
| Rails per bay | Rails per bay. Three is usual for a 1.8 m fence. |
When to use it
Ordering timber for a privacy fence
The board count is the dominant material item and the overlap changes it substantially.
Choosing an overlap
Seeing the cost of each overlap setting makes the privacy trade explicit.
Allowing for timber shrinkage
Boards shrink as they dry, so an overlap that looks generous when built can open to a visible gap.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does the overlap cost?
The same fence at a range of board overlaps.
| Overlap | Boards needed | Effective spacing per board | Total lineal board length |
|---|---|---|---|
| 0 mm | 143 | 140 mm | 257.4 m |
| 25 mm | 174 | 115 mm | 313.2 m |
| 50 mm | 223 | 90 mm | 401.4 m |
| 70 mm | 286 | 70 mm | 514.8 m |
Questions
How much should boards overlap?
Typically 20 to 30 mm, which gives privacy while allowing for timber shrinkage. Green or unseasoned boards can shrink several millimetres across their width as they dry, and an overlap that was minimal at build time can open into a visible gap.
Why is a board-on-board fence more expensive?
Because the overlap means each board covers less than its full width, so more boards are needed for the same length. A 25 mm overlap on 140 mm boards costs about 22% more timber than butting them, and a heavy overlap can double it.
What is the difference from a shadowbox fence?
Shadowbox alternates boards on opposite sides of the rails rather than overlapping them on one face. It looks the same from both sides and allows airflow through, which suits windy sites. Board-on-board puts all boards on one face and blocks the view more completely.
Do I still need a gap between boards?
Board-on-board has no through-gap by design, which is the point. The overlap absorbs shrinkage instead. What matters is that the boards are not forced tight against each other when fixed, so they can move without buckling.
For a standard fence layout, see the fence calculator. For the post holes, see the fence post depth calculator.