The math: rows across, boards along
Deck boards run one direction, so divide the deck's width by the board width plus its gap to get the number of rows — a 12-foot deck with 5½-inch boards and ¼-inch gaps is about 26 rows. Each row is one board (or a scarf joint), so the row count is your board count before waste.
The gap is not optional
Pressure-treated and composite boards both move with water. Gapping at ⅛–¼ inch lets water drain and boards breathe; butted-tight boards cup, trap debris and rot early. Composite brands publish their own required gapping — it varies with temperature at install.
Buy 10% extra, always
The calculator adds a 10% waste factor for crooked boards you reject, end cuts, and the stair or fascia boards you forgot you needed. Straight-lay decks genuinely run close to 10%; herringbone or diagonal patterns need 15–20%, so budget differently.