Deck size in — board count, linear feet, joists and screws out, with the gap between boards accounted for instead of ignored.
The direction the boards run.
Measured across the boards.
Use the actual width, not the nominal size. A "6 inch" board is 5.5 inches.
%
10% covers ordinary offcuts. Use 15% for a diagonal or herringbone layout, which wastes far more at the edges.
Price (optional)
Deck boards to buy
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Deck area—
Rows of boards—
Linear feet of decking—
With waste allowance—
Joists—
Deck screws (approx.)—
Board cost—
Boards + joists—
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The calculation
Deck boards are counted by rows, not by area, and the gap between them is what makes area-based estimates wrong. A 5.5 inch board with a 1/8 inch gap occupies 5.625 inches of deck width. Across a 12 ft deck that is 144 ÷ 5.625 = 25.6, so 26 rows.
Each row is as long as the deck, giving 26 × 16 = 416 linear feet. Add 10% for offcuts and you need 458 linear feet, which at 16 ft stock lengths is 29 boards.
Why the gap is not optional
Pressure-treated lumber arrives wet and shrinks across its width as it dries, so boards laid tight will open up to a gap of their own within a season — unevenly. Composite does the opposite and expands with heat. Either way the gap has to be deliberate: 1/8 inch is standard for dry stock, and many composite manufacturers specify 3/16 or more, with the spacing written into the warranty.
Joists and screws
Joist spacing
Typical use
24 in OC
Minimum for some 2× decking, straight lay only
16 in OC
Standard for most decking, straight lay
12 in OC
Required for 45° diagonal lay and many composites
Screws work out at two per board per joist crossing. A 26-row deck over 13 joists needs about 676 screws — call it a 5 lb box.
The single most useful thing this calculation tells you is not the board count but the width of the final row. Decks rarely divide evenly into board widths, and the last row against the house is where that shows. Landing on a one-inch sliver is unpleasant to fasten and looks like an accident; noticing it before you order gives you the chance to adjust.
Three ways to fix an awkward last row
You can widen the gaps fractionally across the whole deck — spreading a couple of inches over twenty-six rows is invisible. You can rip the first and last boards so the narrow pieces are symmetrical. Or you can adjust the deck width itself by an inch or two while the frame is still on paper, which is easiest of all.
Choose stock lengths that match the deck
A 16 ft deck built from 16 ft boards has no butt joints in the field, which looks better and lasts longer. If the deck is longer than the available stock, plan the joints deliberately: stagger them across a joist, never leave two joints adjacent, and make sure every board end lands on solid framing rather than in mid-air.
Order the whole deck at once
Timber colour, treatment batch and moisture content all vary between deliveries, and composite dye lots vary more. Boards bought three weeks apart can be visibly different on the same deck. Include the waste allowance in the first order rather than topping up later.
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Frequently asked questions
How many deck boards do I need?
Divide the deck width in inches by the board width plus the gap to get the number of rows, multiply by the deck length for linear feet, add 10% waste, then divide by your stock length. A 16 × 12 ft deck using 5.5 inch boards needs about 29 boards of 16 ft.
What gap should I leave between deck boards?
One eighth of an inch is standard for dry or kiln-dried lumber. Wet pressure-treated boards can be laid tight because they shrink as they dry. Composite decking usually needs 3/16 inch or more — check the manufacturer's specification, since warranty terms often depend on it.
How many screws do I need for a deck?
Two per board at every joist crossing. Multiply the number of board rows by the number of joists and double it — roughly 350 screws per 100 square feet at 16 inch joist spacing.
What is the actual width of a 2x6 deck board?
5.5 inches. Nominal lumber sizes are larger than actual ones: a 2×6 measures 1.5 × 5.5 inches and a 5/4×6 is about 1 × 5.5 inches. Always calculate with the actual width or you will under-order.
How far apart should deck joists be?
16 inches on centre is standard for straight-laid decking. Use 12 inches for boards run at 45 degrees and for many composite products, since they are less stiff than timber. Always check the decking manufacturer's span table before framing.