How many deck boards do I need?

Divide the deck width by the board width plus the gap to get the number of rows, then work out how many stock boards each row takes. A 5 × 4 m deck with 144 mm boards and a 5 mm gap needs 4,000 ÷ 149 = 27 rows; at 3.6 m stock a 5 m run takes two boards per row, so 54 boards. That second board is where the waste is, and it is why deck length is worth choosing around your stock length.

Take it from the span tables for your timber grade and section, not from a rule of thumb. 400 mm centres are common under composite decking and where boards run diagonally, 450 mm is typical for softwood, and 600 mm only works with specific products and shorter spans. The right answer depends on the timber, the section, the span and the load — which is why it is an input here rather than something this tool decides.

No, and deliberately. Joist and bearer sizing, post sizing and footing depth come from published span tables and the loading for your site. A calculator that produced a number from a length and a width would be inventing engineering. This tool counts material for the sizes and centres you specify; the sizes come from the tables or your designer.

Around 5 mm for kiln-dried softwood that will still move a little. Composite manufacturers publish a required gap for both the width and the butt joints, and it varies by product and by installation temperature — follow theirs, because it affects the warranty as well as the drainage.

This tool already captures the real source of deck waste by counting boards per row rather than dividing total linear metres, so a 5 m deck on 3.6 m stock is charged for the offcut it actually produces. On top of that, add material for steps, balustrade, picture framing and any angled or herringbone pattern — none of which are in the rectangle you measured.