How the calculation works
The formulas
Direction sets which deck dimension runs along the board. Across the deck, the model includes gaps only between rows. Each row uses whole stock lengths rounded up independently before a waste allowance.
Rows = ceil((cross dimension + gap) ÷ (board width + gap))
Base boards = rows × ceil(run dimension ÷ stock board length)
Purchase boards = ceil(base boards × (1 + waste/100))
Assumptions to check
- Offcut reuse, butt-joint gaps, staggered joints, and perimeter trim are not optimized.
- Joist spans, fixings, loading, and structural capacity are not calculated.
A worked example
A 4 × 3 m deck with 140 mm boards, 6 mm gaps, and 2.4 m stock laid along 4 m needs 21 rows and 42 boards before waste; 10% gives 47 boards.