gizmobench

Deck Board Calculator

Give it the deck, the face width of the board, the gap you are laying to and the length of the stock you are buying, and it returns the rows, the boards and the offcut. The gap is counted between every pair of rows and never after the last one, and the row that does not land flush is shown rather than hidden: you see the width the final board is ripped to, or, if you would rather stop at the last whole board, how much of the span is left bare. Fractions are typed the way a lumber yard prints them, so 1/8, 3/16 and 5 1/2 all read as measurements, and the whole takeoff runs in your browser with no account and nothing uploaded.

12 x 16 ft5.5 in board1/8 in gapDiagonal off
deck      12 ft x 16 ft
board     5.5 in wide, 1/8 in gap
stock     12 ft, along the length, straight lay

rows      35   34 full, last ripped to 3/4 in
boards    39   35 laid plus 4 spare
linear ft 420   of decking laid
offcut    0 ft   nothing dropped
Rows
351 ripped
Boards
39of 12 ft
Linear ft
420
Waste
+10%4 boards
Units
Boards run
Last row
Lay

Type the gap the way the yard prints it: 1/8, 3/16 and 5 1/2 are all read as measurements. Switching the units rewrites the figures already typed rather than clearing them.

  • The last row is ripped to 3/4 in: 34 full boards and the gaps cover 191 1/8 in of the 192 in span.
  • One board of 12 ft covers each 12 ft row.
  • The waste allowance of 10 percent adds 4 boards on top of the 35 laid, rounded up to a whole board.
  • Nothing is dropped: 420 ft bought covers 420 ft laid.
  • 12 x 16 ft, 5 1/2 in, 1/8 gapstraight lay on 12 ft stock
    35 rows, 39 boards, 420 ft
  • Diagonal 45 degreesthe same deck, turned
    43 rows, 44 boards, 409.60 ft
  • Zero gapthe same deck, laid tight
    35 rows, 39 boards, 420 ft
A count of material, and nothing beyond it. Exact arithmetic on the dimensions you enter, including the gap. It estimates material only and makes no span, joist or structural recommendation. The last row is the part most calculators leave out: this one shows the width the final board is ripped to, or how much deck is left bare when only whole boards are laid, so the figure is a decision rather than a rounding nobody sees. The offcut is the stock bought less the decking laid, which is why the stock length is a field.

Common questions

How many deck boards do I need for a 12 by 16 foot deck?
With 5 1/2 inch boards and a 1/8 inch gap, boards running along the 12 foot side, the rows cross the 16 foot side, which is 192 inches. The row pitch is 5.625 inches, so 34 full rows plus their gaps cover 191 1/8 inches and leave 7/8 of an inch. Take one gap out of that and the final board is ripped to 3/4 in, which makes 35 rows in all. On 12 foot stock that is 35 boards and 420 linear feet, and a 10 percent waste allowance rounds it up to 39 boards. Change any of those figures and the count follows.
Does the gap between the boards change how many I need?
Yes, and it is counted at every row. The pitch of a row is the board face plus one gap, so a 6 inch board with a 1/4 inch gap repeats every 6.25 inches: over a 6 foot span that is 11 full rows instead of the 12 you get laying the same boards tight, and the leftover becomes a 3 1/4 inch rip. The gap is only counted between rows, never after the last one, because the deck ends on a board.
What happens to the last row when the deck does not divide evenly?
You choose, and the page tells you what the choice costs. Rip to fit adds a final board cut down to the width that is left, and the width is shown as a fraction when it lands on a sixteenth and as a decimal when it does not. Whole boards stops at the last full row and reports the strip of deck left bare instead. On the 12 by 16 foot example that is a 3/4 inch rip one way, or 7/8 of an inch of bare span the other. When the leftover is narrower than a single gap, the page keeps the whole boards and shows the strip rather than inventing a sliver nobody could cut.
How much extra should I allow for a diagonal deck?
The page adds 15 percent to the length for a 45 degree lay, which is the allowance the trade quotes for the angled cuts at both ends of every row, and it says so beside the number. The rows are counted across the turned span, the length plus the width divided by the square root of 2, and because every row is then a different length the boards are worked out from the total length rather than from rows times stock. That total is an estimate and the page labels it as one; a straight lay is exact arithmetic.
Can it work out joist spacing or how far a board will carry?
No, and it is built not to. This is a material takeoff: it counts rows, boards and offcut for the deck you describe. It holds no span table and sizes no joist, and nothing here is a fixing schedule. Those numbers come from your local building code or from the person specifying the job, and a figure we estimated is not one to build on.
Can I work in metres, and does it remember my deck?
Both. The units control swaps between feet with inches and metres with millimetres, and switching rewrites the figures you have already typed into the new units rather than clearing them, so 12 ft becomes 3.658 m and a 5 1/2 inch board becomes a 139.7 mm one. The deck, the board, the gap, the stock length and the settings are kept in your browser's own storage so the page opens where you left it. Nothing is uploaded, there is no account, and clearing your browser data clears the draft too.

Exact arithmetic on the dimensions you enter, including the gap. It estimates material only and makes no span, joist or structural recommendation.