How many decking boards do I need?

How it works

Board rows are the smallest whole number whose widths plus the gaps between adjacent boards cover the deck width. Their combined run is increased by the chosen cutting allowance and divided by stock length, while joists round up to the maximum permitted centres and crossings determine the fixing count.

• The product installation guide must supply board width, side gap, maximum joist centres, and the number of screws or clips at a crossing.
• Waste is required as a layout decision because butt joints, picture framing, stairs, and unusable offcuts change it.

Worked example

											Deck length m: 4
Deck width m: 3
Board width mm: 145
Board gap mm: 5
Board length m: 3.6
Joist spacing mm: 400
Fixings per crossing: 2
Waste allowance percent: 10
										

												Deck area m2: 12
Board rows needed: 21
Boards to buy: 26
Joists needed: 11
Total joist length m: 33
Fixings needed: 462
											

When to use this

Composite-deck plans take spans and gaps from the product guide, timber cut lists convert rows into stock lengths, and screw-fixed subframes count every board-joist crossing.

Edge cases

• Twenty 145 mm rows with nineteen 5 mm gaps cover 2.995 m, so a 3 m deck needs a twenty-first row; treating every row as if it contributed a following gap gets this boundary wrong.
• A run that is not exactly divisible by the permitted joist centres rounds up and adds the far-end joist, keeping actual spacing at or below the entered maximum.
• Face screws may require two fixings per board-joist crossing while hidden clips use another rule, so the selected system’s count is mandatory.

References