How many deck boards you need comes from two numbers: how many rows fit across the deck, which is the depth divided by the covering width of one board, and how many boards each row eats, which is the row length divided by the stock length. On a 16 by 12 deck that is 29 boards, laid across the 12 foot dimension in 26 rows, using 16 foot stock with a 10 percent allowance. That works out at 416 linear feet over 192 square feet of deck, and the reason it is such a tidy number is the single most useful thing to understand before you finalize the size.
How many deck boards you need, and where the 29 comes from
A 5.5 inch board with a quarter inch gap covers 5.75 inches of deck. Across 12 feet that is 26 rows, and each row is 16 feet long, so each row takes exactly one 16 foot board. Twenty-six boards, plus the waste allowance, is 29.
Rows of decking
rows = deck width in inches ÷ ( board width + gap )
144 ÷ 5.75 = 26 rows across a 12 foot deck
The rest of the frame comes with it: 13 joists at 16 inch centers and 9 footings, for about $831 of decking and joists, or $4.33 a square foot.
The gap is not optional
A quarter inch between boards is what lets water drain and the timber move, and it is why 26 rows cover 12 feet rather than the 27 that a bare 5.5 inch board would suggest. Close the gap up and you gain a row of coverage and lose the drainage, which is a bad trade in every climate. Pressure-treated stock that is still wet from the yard will also shrink as it dries, so a tight gap now becomes a wide one later, and most people lay wet boards nearly touching for exactly that reason.
Why 10 percent waste
Boards get cut short, a few arrive cupped or split, and the last row almost never lands exactly on the edge. Ten percent covers a plain rectangle. It is the allowance that turns 26 rows into a 29 board order, and it is the reason the count is not simply the number of rows.
Why 17 feet costs twice as much as 16
This is the part worth reading before you draw anything. Add a single foot to the length and the board count doubles.
- 16 x 12: 29 boards, $831, $4.33 a square foot
- 17 x 12: 58 boards, $1,484, $7.27 a square foot
Nothing about the deck changed except twelve inches. What changed is that a 17 foot row no longer fits on one 16 foot board, so every one of the 26 rows now needs a second board to finish it, and the offcut from that second board is a foot of decking with nowhere to go.
A 17 foot deck uses the same 58 boards as a 32 foot one. At 32 by 12 the cost is $1,649 against $1,484, so 96 extra square feet costs $165. If you have gone past 16 feet at all, you have already paid for the second board in every row, and the deck may as well be as long as you can use.
The pattern across lengths
Board count steps rather than climbing, and cost per square foot sawtooths with it. On a 12 foot wide deck with 16 foot boards:
- 14 x 12: 29 boards, $804, $4.78 a square foot
- 15 x 12: 29 boards, $818, $4.54
- 16 x 12: 29 boards, $831, $4.33, the best value on the list
- 17 x 12: 58 boards, $1,484, $7.27, the worst
- 20 x 12: 58 boards, $1,525, $6.35
- 24 x 12: 58 boards, $1,566, $5.44
- 32 x 12: 58 boards, $1,649, $4.29
- 33 x 12: 86 boards, $2,279, $5.75
The efficient sizes are the ones that land exactly on a multiple of your board length. Sixteen and thirty-two are the sweet spots here, and seventeen and thirty-three are the two worst places to stop.
The fix is the board, not the deck
If the deck has to be 17 feet, the answer is not to shrink it. It is to buy longer stock.
- 17 x 12 with 12 foot boards: 58 boards, $1,484
- 17 x 12 with 16 foot boards: 58 boards, $1,484
- 17 x 12 with 20 foot boards: 29 boards, $831
Same deck, same dimensions, $653 saved and a joint removed from every row, purely by buying 20 foot boards instead of 16. Decking is commonly stocked up to 20 feet, so this is usually an ordering decision rather than a design compromise, and it is worth asking about before accepting a butt joint down the middle of the deck.
Which way you run the boards decides which dimension matters
Everything above assumes the boards run the length of the deck, so the length is the dimension that has to fit the board. Turn the boards ninety degrees and the roles swap: the width becomes the span each board has to cover, and the length becomes the direction that just adds rows.
That is a real lever on an awkward size. A deck that is 17 feet one way and 12 the other is a disaster with boards running along the 17, and lands neatly if they run across the 12 instead, because 12 fits inside a 16 foot board with room to spare.
It is not a free choice, though. Deck boards have to run across the joists: the American Wood Council’s DCA 6 allows anything from perpendicular to 45 degrees, and requires every piece to bear on at least three joists. So changing the board direction means framing the deck the other way, which changes the joist run, the beam positions and often the footing layout. Decide the direction at the framing stage, not when the decking arrives.
Width behaves normally
All of the drama is in the length. Width just adds rows, in a straight line, with no cliff to fall off. On a 16 foot deck:
- 16 x 8: 19 boards, 17 rows, 6 footings, $606
- 16 x 10: 24 boards, 21 rows, 9 footings, $719
- 16 x 12: 29 boards, 26 rows, 9 footings, $831
- 16 x 14: 33 boards, 30 rows, 9 footings, $921
- 16 x 16: 38 boards, 34 rows, 9 footings, $1,034
So if you want more deck and you are already at 16 feet long, widening is the cheap direction and lengthening is the expensive one. Going from 16x12 to 16x16 adds 64 square feet for about $203. Going from 16x12 to 17x12 adds 12 square feet for $653.
What the count leaves out
These figures cover decking boards and joists. A real order also needs beams and posts, joist hangers and fasteners, concrete for the footings, and the ledger and flashing where the deck meets the house. Railings, stairs and any picture-frame border around the edge are separate again, and a border in particular changes the board math because it adds a perimeter run and cuts every field board short.
Ten percent waste suits a plain rectangle. A deck with angles, a wrapped corner or a step-down needs more, for the same reason a cut-up roof does: every diagonal produces an offcut nothing else will take.
The deck calculator takes the dimensions, the board size and the stock length and returns boards, joists and footings together, which is the quickest way to test whether your planned size lands on the right side of a board-length boundary. The fence calculator handles a railing run priced like fencing, and the concrete calculator covers the footings.
Work out your own deck
Frequently asked
29 boards of 16 foot stock, laid in 26 rows across the 12 foot dimension, including a 10 percent allowance. That is 416 linear feet over 192 square feet, plus 13 joists and 9 footings.
Because a 17 foot row will not fit on a 16 foot board, so all 26 rows need a second board. The count goes from 29 to 58 and the cost per square foot from $4.33 to $7.27, for one extra foot of deck.
Either land the deck length on a multiple of your board length, or buy longer stock. A 17 by 12 deck takes 58 boards on 16 foot stock and 29 on 20 foot stock, a saving of about $653 for the same deck.
26 rows, because a 5.5 inch board plus a quarter inch gap covers 5.75 inches, and 144 inches divided by 5.75 is 26.
Wider, once you are at a board length. Going from 16x12 to 16x16 adds 64 square feet for about $203, while going from 16x12 to 17x12 adds only 12 square feet and costs $653.