| Deck (L × W) | Board width | Gap | Run direction | Required length | Boards | Total (with 10% waste) |
|---|---|---|---|---|---|---|
| 16 ft × 12 ft | 5.5 in | 0.125 in | Along length | 16.000 ft | 26 | 457.600 ft |
| 5 m × 3 m | 140 mm | 3 mm | Along width | 3.000 m | 22 | 72.600 m |
- Required cut length:
Required = Run + 2 × Overhang - Board pitch:
Pitch = BoardWidth + Gap - Boards needed (courses):
Boards = ceil((Cover + Gap) / Pitch) - Total linear length:
Total = Boards × Required - Waste-adjusted total:
TotalWaste = Total × (1 + Waste%/100)
- Select your unit system for consistent inputs.
- Enter deck length and width from your layout.
- Pick the board run direction used on-site.
- Add any trim allowance or overhang you need.
- Enter actual board width and planned gap size.
- Set waste percent based on cuts and defects.
- Choose standard lengths to match supplier stock.
- Press Calculate to view results above.
- Use Download CSV or Download PDF for records.
Measurement inputs that drive accuracy
Deck length and width define the run and cover dimensions. When boards run along the deck length, each course spans the length, while course count is governed by deck width. Enter actual field dimensions, not rounded estimates, and confirm units before calculating. Small input errors multiply across many boards and can shift total linear footage for cleaner, safer installation decisions.
Board width, gap, and course count
Course count is calculated from the effective pitch: board width plus the planned gap. A common 5.5 in board with a 1/8 in gap covers about 5.625 in per course. On a 12 ft cover dimension, that yields about 26 courses. Narrower boards or larger gaps raise counts and increase demand. In humid gardens, a slightly wider gap can improve drainage and drying.
Cut length with trim allowance and overhang
Required cut length equals the run dimension plus two allowances, one at each end. Even a modest 1/4 in trim on both ends adds 1/2 in per board. Across 26 boards, that is over 13 in of extra cutting length. If you plan picture framing or fascia trims, enter the allowance that matches your detailing and cutting method.
Waste factor and purchasing strategy
Waste accounts for end cuts, defects, layout tweaks, and miscuts. Typical waste ranges from 8–15% for simple rectangular decks and can exceed 20% for complex shapes. The calculator applies waste to total linear length and estimates boards to buy when a single standard length meets the cut length. This supports quick budgeting and pickup lists.
Standard lengths, splices, and framing checks
Suppliers stock discrete lengths, so the tool compares required cut length to your listed standards. If no single length fits and splicing is allowed, it suggests a two‑piece plan, used only when seams land on adequate framing. The joist estimate uses run length and spacing to provide a quick check before ordering, then confirm against your framing plan.
FAQs
Does this calculator work for metric projects?
Yes. Switch to Metric and enter dimensions in meters and millimeters. The calculator converts inputs internally and reports required cut length, linear totals, and recommendations in the selected units.
Why does the formula add one gap in the board count?
Boards cover width with gaps between them, but the last board has no trailing gap. Adding one gap to the cover dimension prevents undercounting when using a pitch of board width plus gap.
What waste percentage should I choose?
Use 8–12% for simple rectangles and straight cuts. Choose 12–20% for angled edges, multiple openings, or variable board lengths. Increase waste if you are picky about grain, color, or defects.
How should I enter board width?
Enter actual width, not nominal. For example, many “6-inch” decking boards measure about 5.5 inches. Accurate width improves course count and total linear length, especially on wider decks.
Is the splice suggestion always acceptable?
No. Splices should land on solid framing and follow your local building guidance. Use the suggestion only as a planning aid, then design blocking or adjust lengths to keep joints properly supported.
Why is there a joist estimate?
It provides a quick check using run length and joist spacing. It is not a structural design. Always verify joist count, cantilevers, and blocking against a detailed framing layout.