Enter the deck size, board width, gap, board length and a waste allowance, and get the number of boards needed with boards running along the deck length.
Estimate only, assuming boards run along the deck length and a simple rectangular deck.
The calculation assumes boards run along the deck length. The number of rows is the deck width divided by the board width plus the gap, rounded up, because each board covers its own width plus one gap. Each row needs the deck length divided by the stocked board length, rounded up, boards cut to length. Total boards are the rows multiplied by the boards per row, increased by the waste percentage and rounded up. This is an estimate for a rectangular deck with boards in one direction: picture-frame borders, diagonal patterns, stairs and cut-outs change the count, and boards with defects are a normal reason the waste allowance gets used. Deck area is length multiplied by width and is useful for comparing stain and treatment quantities.
How does the gap change the board count? Each board effectively covers its width plus one gap, so a 5.5 in board with a 0.25 in gap covers 5.75 in of width per row. Wider gaps mean fewer rows across the same deck width.
Why add a waste allowance? Cuts produce offcuts that are too short to use, some boards arrive split, warped or damaged, and end matching wastes length. Ten percent is a common planning allowance for a simple rectangular deck; patterned layouts need more. The result is an estimate, not an exact order quantity.
What if the deck is not a simple rectangle? Break the deck into rectangular sections, calculate each one, and add the results. L-shaped decks, borders and diagonal boards all change how boards are cut, so the estimate is a starting point for those layouts.