Decking Calculator
Calculate how many deck boards you need. Enter deck dimensions and board size to get board count, linear feet, square footage, and fastener estimates.
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Decking Estimate
How to Calculate Deck Board Quantity
Calculating the number of deck boards needed depends on three factors: deck width, board width, and the gap between boards. The process is straightforward but requires attention to the joint gap and waste factor.
- Convert deck width to inches: Multiply the deck width in feet by 12 to get inches.Width (in) = Width (ft) × 12
- Calculate board + gap width: Add the board width and the gap width together.Effective Width = Board Width (in) + Gap (in)
- Divide to find board count: Divide the total deck width in inches by the effective width and round up.Boards = ⌈Width (in) ÷ Effective Width⌈
- Calculate linear feet: Multiply the board count by the deck length to get total linear feet of decking material needed.
Example Deck Calculation
12′ × 12′ deck with 5.5-inch boards and 1/8-inch gap
- Deck width: 12 ft × 12 = 144 inches
- Effective board width: 5.5 in + 0.125 in = 5.625 in
- Boards needed: 144 ÷ 5.625 = 26 boards
- With 10% waste: 26 × 1.10 = 29 boards
- Linear feet: 29 boards × 12 ft = 348 linear feet
- Fasteners: 29 × 18 screws per board = 522 screws
Common Mistakes in Decking Estimation
Forgetting the gap between boards
Deck boards need gaps for drainage and expansion. Treated lumber needs 1/8 inch gaps; hardwood needs 1/4 inch. Failing to account for the gap results in ordering too few boards and installing them too tight, which traps moisture and causes cupping.
Ignoring board shrinkage
Pressure-treated lumber is often installed wet and shrinks as it dries. A 5.5-inch board can shrink to 5.25 inches over time. This creates wider gaps than expected. Account for this by not overspacing during installation.
Not planning diagonal layouts
Diagonal decking requires significantly more material — typically 15-20% more than straight-laid boards. The mitered ends at the perimeter also increase waste. Adjust your waste factor to 15% for diagonal patterns.
Frequently Asked Questions
How many deck boards for a 12x12 deck?
For a 12x12 deck using 5.5" boards with 1/8" gap: (144/5.625) = 26 boards. Add 10% waste = 29 boards.
What gap should I leave between deck boards?
1/8" for treated lumber, 1/4" for hardwood. Treated lumber shrinks as it dries, so gaps will widen over time.
How many fasteners per deck board?
Two fasteners at each joist intersection. For 16" OC joists on a 12 ft board, that's about 18 fasteners per board.