How deck board calculator works
Decks are laid out one direction at a time: boards run parallel to the deck's length, rows span the width, and each row holds as many whole boards as fit. The calculator computes rows from the installed board width plus the gap between boards — 5/4 pressure-treated decking is 5½ in wide, with a 3/16–1/4 in gap for drainage, and composite boards typically run 5½ in too.
Rows use the exact coverage formula floor((width + gap) ÷ (board + gap)), so the estimate never invents a phantom last board. If your boards come shorter than the deck, enter the stock length and each row splits into the fewest full-length pieces — a 16 ft deck built from 8 ft boards is two per row, with butt joints you stagger by eye. A waste allowance (10% by default) then rounds the purchase up, covering rim off-cuts, the one warped board in the pile, and the plank you mis-cut twice.
Structure and fasteners fall out of the same numbers. Joists run perpendicular to the boards at 12, 16 or 24 inches on center, plus the rim joist at the far end. Every board-to-joist crossing gets two screws — the deck-building standard that stops cupping and squeaky treads — and the screw count converts to pounds at roughly 350 screws per pound of 1⅝–2½ in decking screws.