Ordering decking is more than dividing floor area by board area, because the expansion gap between boards and the joins along each run both change how many boards you actually need. This calculator handles both, then adds a cutting-waste allowance.
How it works
pitch = board width + expansion gap
rows = ceil(deck width / pitch)
boards per row = ceil(deck length / board length)
boards = rows × boards per row
to buy = ceil(boards × (1 + waste%))The gap matters: laying 144 mm boards with a 5 mm gap gives a 149 mm pitch, so a 3 m wide deck needs 21 rows rather than the 20 that ignoring the gap would suggest.
Worked example
A 4 m long, 3 m wide deck with 3.6 m long, 144 mm wide boards, a 5 mm gap and 10 percent waste:
- Pitch = 149 mm, rows = ceil(3.0 / 0.149) = 21
- Boards per row = ceil(4.0 / 3.6) = 2
- Boards = 21 × 2 = 42, plus 10 percent = 47 boards
What to watch
Joists come first. Boards should be supported so that end joins land on a joist. Plan your joist spacing before ordering, as it can change how the boards break.
Direction changes waste. Running boards diagonally, or framing the edges with a picture-frame border, increases offcuts — bump the waste percentage up accordingly.