Calculate how many deck boards you need based on deck size, board width and the gap between boards.
Divide the deck width by the board width plus gap to get the number of rows, then multiply by the number of board lengths needed to span the deck length. This calculator does it for you and adds waste.
A gap of 3–6 mm (1/8 to 1/4 inch) is recommended for drainage and to allow the timber to expand and contract. Composite boards often need slightly larger gaps — check the manufacturer guidance.
Allow 5–10% extra for cuts, offcuts and mistakes. Use the higher end for diagonal or herringbone patterns.