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Decking Calculator
Decking is counted in boards, but boards are laid in rows with a gap, over joists on a spacing — so the honest count comes from the layout, not the square footage. Enter the deck and your board, and get the rows, the boards, the joists under them, and the screws that hold it all down.
Your results
Shopping list — buy this much
Joists listed are the count spanning the deck's length at your spacing, each cut to the deck's width. Rim joists, beams, posts and footings depend on your frame design and local code — this calculator covers the decking package, not the structure under it.
How it works
Boards = rows × boards per row × (1 + waste %)
Joists = deck length × 12 ÷ spacing + 1
Screws = rows × joists × 2 per crossing (+5%)
Two traps this layout math avoids. First, the nominal width trap: a 2×6 is 5.5 inches wide, and planning with 6 inches undercounts a 12 ft deck by two full rows. Second, the square-foot trap: decking isn't sold by area, so "192 sq ft of decking" isn't an order — boards in lengths that fit your deck are. If your deck is 14 ft long and boards come in 12s and 16s, every row is either a 16 cut down or a splice over a joist; the boards-per-row step handles that.
Screw count assumes two screws (or two hidden-fastener clips) at every board-joist crossing. Diagonal layouts cross more joists per board — bump the count by about a third, and the waste to 15%.
Worked example
A 16×12 ft deck (192 sq ft) with 5.5 in boards and a 1/4 in gap: each row covers 5.75 in, so 144 in of width needs 26 rows. In 16 ft boards each row is one board, so 26 boards — 29 with 10% waste, $1,102 at $38 a board. Underneath: joists every 16 in across the 16 ft length means 13 joists, and 26 rows × 13 joists × 2 screws × a little extra = about 710 screws — buy the 5 lb (or 10 lb) bucket, not the 1 lb box.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
With standard 5.5 in boards and a 1/4 in gap, 26 rows of 16 ft boards — 29 boards with a 10% waste allowance. In 12 ft boards you'd need two per row (spliced over a joist): 52 plus waste. Board length choice changes the count more than any other input.
How do I calculate linear feet of decking?
Rows × deck length. Or from area: linear feet = sq ft × 12 ÷ (board width + gap, in inches). For 5.5 in boards at a 1/4 in gap, that's about 2.09 linear feet per square foot — the multiplier lumber yards use when they quote decking by the foot.
How many screws does a deck need?
About 3.5–4 screws per square foot of deck with 5.5 in boards on 16 in joists — two at every crossing, plus spares. A 200 sq ft deck uses roughly 700–800. Hidden fastener systems replace the screws with clips at the same crossings, so the count barely changes.
What gap should I leave between deck boards?
About 1/4 in for composite and kiln-dried wood. Fresh pressure-treated lumber is wet and swells — lay it tight (or with a nail's width) and it will shrink to roughly a 1/4 in gap as it dries over the first season. Always follow a composite manufacturer's spec; their clips usually set the gap for you.
Is composite or wood decking cheaper?
Wood is cheaper to buy — often half the per-board price of composite — and composite is cheaper to own: no annual sealing, no splinters, 25-year warranties. The calculator's cost field is the buy price either way; set it to the board you're actually pricing and compare the totals, then price your weekends honestly.
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About the author
Harrison Alley is a web publisher and data enthusiast based in Dallas, Texas. He built Urbarama’s remodeling calculators and maintains them.
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