How many deck boards do I need?
A 12 × 12 ft deck needs about 43 boards of 5.5 in × 8 ft decking, including a 10% waste allowance and the small gap between boards. Here's how to size the decking for any deck.
The formula
Boards = deck area × (1 + waste %) ÷ board coverage
Board coverage accounts for the gap left between boards for drainage and expansion:
Board coverage = (board width + gap) ÷ 12 × board length (in feet)
A worked example
For a 12 × 12 ft deck using 5.5 in boards, 8 ft long, with a 1/8 in gap:
- Deck area: 12 × 12 = 144 sq ft
- Board coverage: (5.5 + 0.125) ÷ 12 × 8 = 3.75 sq ft per board
- Add 10% waste: 144 × 1.10 = 158.4 sq ft
- Divide: 158.4 ÷ 3.75 = 42.2 → 43 boards
Skip the math
Our deck calculator gives you the boards, linear feet, screws, and cost from your deck size and board dimensions.
Open the Deck CalculatorWhat about screws and framing?
Budget about 3.5 deck screws per square foot — roughly two per board at every joist, with joists spaced 16 inches apart. The decking boards are only the surface, though: the joists, beams, posts, and footings underneath are sized separately based on your span and local building code. If you're unsure about the structure, check with your building department before you buy.
Choosing board sizes
The most common decking is "5/4 × 6," which actually measures about 5.5 inches wide. Wider boards cover faster with fewer fasteners; narrower boards suit smaller or more detailed decks. Longer boards reduce butt joints but are heavier to handle.