How deck board width and gaps affect board count
Deck boards cover a width with boards and the gaps between them. The gap is part of the layout, so it belongs in the material count.
Use the effective board pitch
The effective pitch is board width plus the planned gap. A row of boards has one fewer internal gaps than boards, which matters when the final row is close to the deck edge.
[ 140 mm board ][ 6 mm gap ][ 140 mm board ]
Board rows
rows cover the deck width using board width + gap, adjusted for the final boardTry the actual gap
Use the gap recommended for the board product and conditions. With the same deck width, a 3 mm, 6 mm, or 8 mm gap can produce a different row count. Do not choose a gap only to lower the material estimate.
Direction also changes board runs
Running boards along the long dimension may reduce joins; running them the other way changes the number of board lengths per row. Test both permitted directions, then account for trimming, layout features, and the framing plan separately.