Buying composite decking by the board is easy to get wrong because the deck is measured in square feet but the boards are sold in lineal feet. This calculator bridges the two, accounting for the board width, the expansion gap, the layout angle, and the joist spacing that drives your fastener count.
How it works
Every board occupies a strip of the deck equal to its face width plus the gap to the next board. Working in feet, the lineal footage of board is the deck area divided by that strip width:
strip_width_ft = (board_width_in + gap_in) / 12
lineal_ft = deck_area_ft2 / strip_width_ftA diagonal layout multiplies the result by 1.15 to cover the extra cut-off. The board count is the lineal feet divided by your chosen board length, rounded up. Fasteners are estimated as one hidden clip per board for each joist it crosses, so tighter joist spacing raises the clip count.
Tips and example
For a 320 sq ft deck with 5.5 in boards and a 3/16 in gap, each board claims a 5.6875 in strip, so you need about 675 lineal feet, or 43 boards at 16 ft. Always round up and add overage: order five to ten percent extra for square decks and more for angled or multi-level designs. If you plan a picture-frame border, add one board per side cut to the deck perimeter, and check your specific decking brand for its recommended gap and clip system before buying.