Board-on-Board Fence Calculator
Enter your fence length, picket width, and overlap to instantly find how many boards, posts, and rails your board-on-board fence project requires. Switch between imperial and metric units, add a cost-per-board price for a total material estimate, and see the full step-by-step working so you can verify the numbers before you buy.
What is a board-on-board fence?
A board-on-board fence (sometimes called a "shadowbox" fence) alternates vertical pickets on opposite sides of the horizontal rails, with each picket overlapping the ones flanking it. Because there is no single gap running straight through the fence, you get a fully private view from any angle while still allowing airflow and some light to pass through. The double-layer design also makes the fence look identical from both sides, which is useful on a property boundary shared with a neighbour.
How the picket count is calculated
The core formula is:
pickets = (fence length - picket width) / (picket width - overlap) + 1
The first board takes one full picket width of space. Every board after it only advances by the effective spacing, which is the picket width minus the overlap. For a 100-ft fence using 5.5-in boards with a 1.5-in overlap, the effective spacing is 4 in (0.333 ft), giving about 299 raw boards before waste. Rounding up plus a 10% waste factor brings the final order to around 329 boards.
A waste allowance of 5-15% covers boards with knots, splitting at cut ends, and alignment losses at gates or corners. Ten percent is the most common rule of thumb.
Posts, rails, and post depth
The number of posts equals the number of sections plus one: a 100-ft run with 8-ft spacing needs 12 sections and 13 posts. Each section gets 2 rails for fences up to 5 ft tall, 3 rails for fences 5-8 ft, and 4 rails for taller structures.
Post depth follows the one-third rule: bury one-third of the total post length. For a 6-ft fence the post should be at least 9 ft long (6 ft above grade, 3 ft below). If you are setting posts in concrete, the diameter of the hole should be three times the post width, and the bottom 6 inches should be gravel for drainage.
Choosing the right overlap
Overlap controls privacy, board count, and long-term performance. A common starting point is 24% of the board face width (e.g. 1-3/8 in on a 1x6). Too little overlap (under 1 in on a 1x6) risks visible gaps after the wood dries; too much wastes material without a proportional gain in privacy.
For pressure-treated pine, which is the most common material, plan for seasonal width changes of 1/8 to 3/16 in per board. An overlap of 1.5 in gives comfortable clearance even after shrinkage. Cedar and redwood are more stable but still benefit from a 1-in minimum overlap.
Cost estimation
Material cost is the product of quantity and unit price for pickets, posts, and rails. Labour, concrete for post footings, hardware (screws, post caps, gate hinges), and any stain or sealer are separate line items not included here.
A rough regional benchmark: pressure-treated 1x6x6 pickets run $2-$5 each, 4x4x9 posts $10-$20 each, and 2x4x8 rails $5-$10 each. Prices vary significantly by region and lumber grade. Always get a current quote from your local supplier before committing to a budget.
Typical board-on-board overlap by board width
| Nominal size | Actual face width | Recommended overlap | Effective spacing |
|---|---|---|---|
| 1x4 | 3.5 in | 0.84 in (7/8 in) | 2.66 in |
| 1x6 | 5.5 in | 1.32 in (1-3/8 in) | 4.18 in |
| 1x8 | 7.25 in | 1.74 in (1-3/4 in) | 5.51 in |
| 1x10 | 9.25 in | 2.22 in (2-1/4 in) | 7.03 in |
Recommended overlap is roughly 24% of board face width. More overlap improves privacy but increases board count.
Frequently asked questions
How many pickets do I need for a 100-foot board-on-board fence?
With standard 1x6 boards (5.5-in actual width) and a 1.5-in overlap, you need about 299 boards for 100 ft of fence. Adding a 10% waste buffer brings that to 329 boards. The exact number depends on your overlap setting.
What is the standard overlap for a board-on-board fence?
About 1 to 1.5 inches (2.5 to 3.8 cm) for a 1x6 board, which is roughly 20-27% of the 5.5-in face width. A good rule of thumb is to multiply the actual board width by 0.24 to get the minimum recommended overlap.
How far apart should fence posts be for a board-on-board fence?
8 feet center-to-center is the most common spacing for a standard wood privacy fence. You can go up to 10 ft with beefy 4x4 posts and extra-thick rails, but wider spacings increase rail sag and reduce overall rigidity.
How deep do fence posts need to be buried?
The standard rule is one-third of the total post length. For a 6-ft fence, buy 9-ft posts and set them 3 ft deep. In regions with deep frost, the bottom of the post should go below the frost line (often 36-48 in in northern US climates) to prevent heaving.
How many rails per section does a 6-foot fence need?
Three rails is the recommended minimum for a 6-ft board-on-board fence: top, middle, and bottom. Two rails are sufficient for shorter fences (4-5 ft). Tall fences over 8 ft benefit from four rails to prevent pickets from bowing outward.
Why does board-on-board use more wood than a standard privacy fence?
Because every other board is on the opposite side of the rail and partially overlaps the boards flanking it, you effectively cover the fence run twice: once on the front face and once on the back. A standard dog-ear privacy fence with no overlap uses fewer boards but shows gaps from an angle.
Can I use this calculator for a shadowbox fence?
Yes. A shadowbox fence is the same construction: alternating pickets on opposite rail faces with overlap. Enter your measurements the same way. The only difference is that a true shadowbox sometimes uses equal spacing between pickets rather than overlap; if that is your design, set the overlap to the gap width rather than a physical overlap.