Decking Calculator

Divide the deck area by the area one board covers (board length × [board width + gap]), then add waste. 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. The calculator handles the widths, the gap and the allowance for you.

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. It is the same as counting rows (deck width ÷ effective board width) and boards per row (deck length ÷ board length). Enter your figures and the calculator shows the rows and total boards, plus the linear feet (metres) to order.

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 for their product and for end-to-end joints — follow it, as a wider gap means slightly fewer boards.

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 — the calculator recalculates both when you change the spacing.

About two screws per board at every joist it crosses — roughly 350 screws per 100 sq ft (9 m²) at 16 in joist spacing. The calculator multiplies your board rows by the number of joists by two. Hidden-fastener systems use clips instead of face screws; check the maker’s coverage if you use them.