Deck Board Spacing Calculator
Deck boards are laid across the width with a small gap between each for drainage and seasonal movement. The number of boards is set by the deck width, the actual board width (a nominal 6-inch board is about 5.5 inches), and the gap you leave. This calculator divides the width by the combined board-plus-gap pitch to give the board count, then works back to the exact gap that makes the boards land edge to edge with a full final board. Gap size depends on your material and its moisture content, so enter the value from your board manufacturer's installation instructions.
Deck board spacing formula
Pitch = board width + gap
Exact boards = (deck width + gap) / pitch
Boards needed = ceil(exact boards)
Even gap = (deck width - boards * board width) / (boards - 1)
Total coverage = boards * board width + (boards - 1) * even gap
A run of N boards has N minus 1 gaps between them. Adding one gap to the deck width before dividing accounts for the missing trailing gap. The even gap spreads any leftover width across all the interior gaps so the last board is full width.
Deck board context
- Nominal board sizes are larger than actual: a 2x6 is about 1.5 by 5.5 inches once surfaced.
- Gaps allow drainage, airflow, and seasonal expansion; follow the board maker's gap spec.
- Wet pressure-treated boards shrink as they dry, so installers may gap them tighter.
- Composite boards move with temperature; gap them per the manufacturer temperature schedule.
- Distribute leftover width evenly across gaps rather than leaving a narrow final board.
Deck board spacing: frequently asked questions
How do I calculate the number of deck boards?
Each board plus one gap occupies the board width plus the gap. Divide the deck width by that combined pitch (board width plus gap) to get the number of boards. The last board usually has no trailing gap, so the exact fit is (deck width plus one gap) divided by (board width plus gap). Round up and rip the final board to fit.
What gap should I leave between deck boards?
Gap size depends on the material and its moisture content when installed. Pressure-treated lumber that is still wet shrinks as it dries, so it is often laid tight or with a thin gap; kiln-dried wood and composites are typically gapped 1/8 to 1/4 inch. Always follow the board manufacturer's installation instructions, which this calculator lets you enter directly.
Why leave a gap at all?
Gaps let water drain off the deck, allow air to dry the boards, and give room for seasonal expansion. Without gaps, trapped water promotes rot and boards can buckle as they swell. Composite decking expands and contracts with temperature, so its gaps are set by the maker's temperature-based schedule.
How do I get even gaps across the deck?
Calculate the exact number of boards, then distribute any leftover width evenly across all the gaps rather than dumping it into the last board. This calculator reports the adjusted gap that makes the boards land exactly edge to edge, so the final board is full width instead of an awkward sliver.
Does board orientation change the count?
Yes. Boards run across the joists, so the count is driven by the deck dimension perpendicular to the board length. Use that span as the deck width input. The board length covers the other direction, and you size board length and joist span separately based on the structural span tables for your lumber.
Official sources
- American Wood Council: American Wood Council (codes and span standards).
- International Code Council: International Residential Code (deck provisions).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.