Deck board calculator
The gap between boards is the detail that decides the count — a quarter inch across twenty rows adds up to most of an extra board.
Gaps, and why they are not optional
Decking needs space between boards for drainage and for the wood to move with the seasons. A quarter inch is typical for dry lumber. Pressure-treated boards delivered wet are a special case: they shrink as they dry, so they are often laid nearly tight and open up to roughly the right gap on their own. Composite decking follows the manufacturer's spacing exactly, since it expands along its length rather than across.
Because each board plus its gap covers slightly more than the board's own width, the effective coverage figure is what drives the row count. That is why entering the nominal six-inch width instead of the actual 5.5 inches would leave you a row short.
Joist spacing of 16 inches on centre is standard for most decking laid perpendicular to the joists. Diagonal layouts need 12 inches, because the board spans further between supports — and composite boards often specify tighter spacing than timber. The screw count assumes two per board at every joist crossing, which is the normal fastening pattern.
Questions
What gap should I leave between deck boards?
About a quarter inch for dry lumber. Wet pressure-treated boards can go nearly tight since they shrink as they dry. Composite decking follows the manufacturer's figure, which is usually larger.
Does board direction change the count?
It changes which dimension the boards span. This assumes boards run along the length, so the width determines how many rows you need. Swap the two inputs for the other orientation.
Why 16 inch joist spacing?
It is the standard for decking laid perpendicular to the joists. Diagonal patterns need 12 inches because the board spans a longer distance between supports.