Deck boards come down to area divided by the area one board covers — but the “board” is really its width plus the gap between boards. Get that right, add waste, and the joists and screws follow from the deck size. Here is the whole method.
Counting the boards
Divide the deck area by the area one board covers: board length × (board width + gap). A 20 × 13 ft deck (260 sq ft / 24 m²) in 12 ft × 5½ in boards with a ¼ in gap covers about 5.7 sq ft per board — roughly 46 boards, so order about 51 with 10% waste.
Decking Calculator
Enter the deck size and board dimensions for the boards, joists and screws — plus the linear feet to order.
The gap between boards
Usually ⅛–¼ in (3–6 mm) for drainage and expansion. Kiln-dried timber is often laid tight (⅛ in) because it shrinks as it dries; composite and wet timber get the wider gap. Composite makers specify an exact gap — follow it, as a wider gap means slightly fewer boards.
Joist spacing
16 in (400 mm) on centre is standard for straight-laid timber decking. Drop to 12 in (300 mm) for composite boards (they sag more), for diagonal or herringbone layouts, or where the manufacturer requires it. Closer joists means more joists and screws.
How many screws
About two screws per board at every joist it crosses — roughly 350 screws per 100 sq ft (9 m²) at 16 in joist spacing. Hidden-fastener systems use clips instead of face screws; check the maker’s coverage if you use them.
Working from square footage
Same method, two steps: total the linear length of decking (deck area ÷ effective board width, where effective width = board width + gap), then divide by your board length and round up. That is the same as rows (deck width ÷ effective width) times boards per row.