Deck Board Calculator
Enter your deck size, board width, the gap between boards, and the board length to find how many deck boards you need. Boards run along the length; the gap is the air space left between them, usually 1/8 inch.
How to calculate deck board
Each row of decking takes up its own width plus the gap left between boards (about 1/8 inch for dry lumber, 1/4 inch for wet-treated). Divide the deck width by that combined figure and round up to get the number of rows. Multiply the rows by the deck length for the total linear feet of decking, divide by the length of one board, and add 10% for cutting waste and the odd bad board.
Rows = Deck width ÷ (board width + gap) · Boards = (Rows × deck length ÷ board length) + 10%
Worked example
A 16 ft × 12 ft deck with 5½-inch boards, a 1/8-inch gap, and 16 ft boards.
- Row width = 5.5 + 0.125 = 5.625 in = 0.469 ft
- Rows = 12 ÷ 0.469 = 25.6 → 26 rows
- Linear feet = 26 × 16 = 416 ft
- Boards = 416 ÷ 16 = 26, + 10% waste = 29
Result: 26 rows, 26 boards (29 with a 10% waste allowance)
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap between boards to get the number of rows, multiply by the deck length for linear feet, then divide by the board length and add 10% for waste. A 16 × 12 ft deck with 5½-inch boards needs about 26 boards, or 29 with waste.
What gap should I leave between deck boards?
Leave about 1/8 inch between kiln-dried or composite boards, and up to 1/4 inch for wet pressure-treated lumber, which shrinks as it dries. The gap lets water drain and air circulate so the deck lasts longer.
Does the gap change how many boards I need?
Yes. A wider gap means each board covers a little more width, so you fit fewer rows across the deck and need slightly fewer boards. This calculator folds the gap into the row width, so the count reflects the spacing you actually plan to use.
How much extra decking should I buy for waste?
Add about 10% for a straight layout to cover end cuts, off-cuts, and the occasional warped or damaged board. Diagonal or picture-frame patterns waste more, so allow 15% for those.
How is this different from a deck calculator?
This tool focuses on the board count and lets you set the gap between boards, so the row count is exact. The full deck calculator also estimates joists and screws for the whole framing job. Use both together when planning a build.