Free Deck Board Count calculator with clear step-by-step results.
Board count is driven by pitch - board width plus the drainage gap - not by width alone, and ignoring the gap on a large deck can leave you a row short. Rows are multiplied by the stock lengths needed to span the deck, then rounded up with a waste allowance for cuts and culls.
Rows required
Rows = ceil(deck width / (board width + gap))
Boards to buy
Boards = ceil(rows x ceil(deck length / stock length) x (1 + waste))
Four to six millimetres for kiln-dried timber and composite. Green or wet-treated timber shrinks, so it can be laid tighter at 2-3 mm and will open up as it dries.
Yes, and stagger them over joists so no two adjacent rows butt at the same point. That is part of why a 10% waste allowance is realistic rather than generous.