Board-on-Board Fence Calculator
Calculate materials for a board-on-board privacy fence: boards, posts, rails, concrete, fasteners, and costs with visual cross-section diagram.
About
Board-on-board fencing (also called shadow-box or good-neighbor fence) alternates pickets on opposite sides of the rail so each board overlaps its neighbor by a configurable amount, typically 12mm to 38mm (0.5 - 1.5in). Miscounting boards is the most common source of cost overruns: because every section carries roughly 1.6× to 2× the pickets of a flat-face fence, a small overlap error compounds across hundreds of boards into dozens of extra pieces or, worse, gaps that defeat the privacy purpose. This calculator applies the overlap geometry precisely, accounts for post-hole concrete volume using V = πr2h, and adds a user-adjustable waste factor because lumber defects and field cuts are unavoidable. It assumes standard residential post spacing of 6 - 8ft on center and 2 or 3 horizontal rails per bay.
Limitations: the tool does not model grade changes (slopes), wind-load engineering, or local building code setback requirements. For sloped terrain, measure each bay individually and sum the results. Pro tip: always confirm the property line with a survey pin before setting corner posts. The cost section uses user-entered unit prices because lumber markets fluctuate seasonally by 15 - 30%.
Formulas
The number of posts along a straight run of total length L with on-center spacing s is:
The clear opening between posts (bay width) is:
where tpost is the post thickness (nominal 3.5in for a 4×4). For board-on-board, boards are placed on alternating sides of the rail. If each board has actual width wb and the overlap between a front board and back board is o, the effective pitch (center-to-center of same-side boards) is:
Boards on one side per bay:
Total boards per bay (both sides):
Post hole concrete volume per post, using hole diameter dhole and depth h:
The second term subtracts the post volume from the hole. Fastener count assumes 2 screws per board per rail intersection. Total materials are multiplied by (1 + wwaste) where wwaste is the waste factor (typically 0.10 or 10%).
Where: L = total fence length, s = post spacing on center, tpost = post thickness, wb = actual board width, o = board overlap, dhole = post hole diameter, h = post hole depth, wwaste = waste factor decimal.
Reference Data
| Component | Typical Size (Imperial) | Typical Size (Metric) | Common Species | Notes |
|---|---|---|---|---|
| Fence Post | 4×4×8ft | 89×89×2440mm | Pressure-treated pine, Cedar | Set 24 - 36in deep |
| Fence Post (tall) | 4×4×10ft | 89×89×3050mm | Pressure-treated pine, Cedar | For fences 7 - 8ft tall |
| Top/Bottom Rail | 2×4×8ft | 38×89×2440mm | Pressure-treated SPF | Horizontal stringers |
| Mid Rail (optional) | 2×4×8ft | 38×89×2440mm | Pressure-treated SPF | Recommended for fences ≥ 6ft |
| Picket (narrow) | 1×4×6ft | 19×89×1830mm | Cedar, Redwood, PT Pine | Actual width ≈ 3.5in |
| Picket (standard) | 1×6×6ft | 19×140×1830mm | Cedar, Redwood, PT Pine | Actual width ≈ 5.5in |
| Picket (wide) | 1×8×6ft | 19×184×1830mm | Cedar, PT Pine | Actual width ≈ 7.25in |
| Post Cap | 4×4in cap | 100×100mm | Cedar, Copper, Vinyl | Decorative & weatherproofing |
| Concrete (fast-set) | 50lb bag | 22.7kg bag | Portland cement mix | ≈ 0.375ft³ per bag |
| Gravel (base) | 50lb bag | 22.7kg bag | Crushed stone №57 | 3 - 4in drainage bed |
| Screws (exterior) | №8 × 2.5in | 4×65mm | Stainless / coated steel | 2 per board per rail |
| Nails (hot-dipped galv.) | 8d ring-shank | 3.3×65mm | Galvanized steel | Alternative to screws |
| Post-hole diameter | 10 - 12in | 250 - 300mm | - | 3× post width rule of thumb |
| Post embedment depth | 24 - 36in | 600 - 900mm | - | Below frost line in cold climates |
| Stain/Sealer coverage | 200 - 400ft²/gal | 5 - 10m²/L | Oil or water-based | Board-on-board uses ≈ 1.8× flat fence area |