How Many Deck Boards Do I Need?
Counting deck boards looks simple until you factor in the gap between boards, the board lengths you can buy, and the joists underneath. Here is how to get the number right.
By the Calculate My Reno Team / Published
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Open the Decking Calculator
Working out deck boards is area divided by what one board covers, plus waste - but three details change the answer: the gap between boards, the board lengths you can buy, and the joists underneath.
The formula
boards = ceil( deck area ÷ (board length × board width) × wastage )
The catch is that “board width” for coverage purposes includes the gap between boards - each board effectively covers its own width plus one gap. The decking calculator handles this, along with joist count and total board run, so you do not have to.
Why the gap matters
You never butt deck boards tight together. A gap lets water drain and air circulate, and timber boards shrink as they season. A typical timber gap is about 5 mm (3/16 inch); composite gapping is set by the manufacturer and depends on the temperature when you install. Because each board plus its gap covers slightly more deck than the board alone, the gap reduces the board count a little - but it is real and worth including.
Worked example
A 12 ft × 12 ft deck, 5.5-inch boards run straight, 10% wastage:
- Area: 12 × 12 = 144 sq ft.
- One 12 ft board covers 12 × (5.5 ÷ 12) = 5.5 sq ft.
- Boards before waste: 144 ÷ 5.5 = 27; add 10%: about 30 boards.
Choose board lengths that fit your deck
Buying the right lengths is how you cut waste. A 20 ft deck run wants 20 ft boards, not three 16 ft pieces with offcuts at every joint. Match board length to your longest deck dimension where you can, and stagger the joints that remain.
Don’t forget joists and fixings
The board count is only the surface. You also need:
- Joists - the decking calculator returns the joist count from your spacing (16 in straight, 12 in for diagonal or many composites).
- Fixings - hidden fasteners for grooved composite boards, or screws for timber and square-edge boards.
Choosing a composite brand? See Trex vs TimberTech vs Fiberon. Still deciding between composite and timber? Read composite vs wood decking.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
A 12 ft × 12 ft deck is 144 square feet. With standard 5.5-inch-wide boards run straight, one 12 ft board covers about 5.5 sq ft, so you need about 27 boards plus a 10% wastage allowance - roughly 30 boards. The exact count depends on board width, the gap between boards and your layout, so run your figures through the decking calculator.
What gap should I leave between deck boards?
For timber, leave about a 5 mm (3/16 inch) gap for drainage and air flow - boards also shrink as they dry. For composite, follow the manufacturer's gapping, which varies with temperature at install (often a few millimetres side-to-side and more end-to-end). The gap slightly increases the number of boards because each board covers a little less width.
How many joists does a deck need?
Joist spacing is usually 16 inches on centre for boards laid straight, and 12 inches for a diagonal layout or for composite boards that require closer support. The number of joists is the deck length divided by the spacing, plus one. Closer spacing means a firmer deck and more joists.