How many deck boards for a 12 × 26 ft deck?

2 × 6 pressure-treated boards

55× 20 ft

26 rows across 12 ft — 710 linear ft including 5% waste

Boards to buy	55× 20 ft
Linear feet676 ft plus 5% waste	710ft
Rows of decking12 ft ÷ 5.625″ repeat	26rows
Board repeat5.5″ actual width + 0.125″ gap — not the nominal size	5.625in
Deck area12 × 26 ft	312ft²
Joists crossedat 12″ o.c. over 26 ft	27joists
Deck screwstwo per board-joist crossing — about 5 boxes of 350	1,404screws
Layoutboards run perpendicular to the joists — least waste	straight
Butt joints26 ft run exceeds the 20 ft board — stagger the joints and land every one on a joist	Yes

Notes

Nominal width is not actual width. A "2 ×" board measures 5.5″ across. With a 0.125″ gap the repeat is 5.625″, which is what decides the row count — 26 rows here. Working from the nominal size undercounts by close to a tenth.
Thicker and stiffer; spans further between joists. The gap is not optional in either direction: too tight on composite and it buckles in July; too wide on wet treated lumber and you get gaps you can drop a phone through by autumn.
Straight layout, 5% waste — boards run perpendicular to the joists — least waste. This is the least wasteful way to lay a deck, and the fastest.
The 26 ft run is longer than a 20 ft board, so there will be butt joints. Stagger them so no two adjacent rows join over the same joist, and double the joist under every joint — a butt joint on a single joist gives each board about ¾″ of bearing.
Fasten with two screws at every board-joist crossing — 1,404 screws here. One screw lets the board cup and lift at the corner, and it is the single most common reason a deck looks tired after two summers.
Lay the boards bark-side up if you can read the end grain. A board cups away from the bark side, so bark-up sheds water off the crown instead of holding it in a trough.

Nominal width is not actual width

This is where board counts go wrong. A 2 × 6 treated board measures its real width, and with the gap it needs the repeat is 5.625″ — which is what actually decides the row count.

12 ft of deck at that repeat is 26 rows, and 55 boards once 5% waste for the straight layout is added.

Working from the nominal size undercounts by close to a tenth.

The gap is not optional in either direction

Treated lumber is sold wet and shrinks. Composite expands with heat. Gap too tight on composite and it buckles in July; gap too wide on wet treated lumber and by autumn you have gaps that swallow a phone.

Screws

1,404 — two at every board-joist crossing, across 27 joists. One screw per crossing lets the board cup and lift at the corner, and that is the single most common reason a deck looks tired after two summers.

Read the end grain

Lay boards bark-side up. A board cups away from its bark side, so bark-up sheds water off the crown rather than holding it in a trough.

Butt joints

Where the run exceeds a standard board length, stagger the joints so no two adjacent rows meet over the same joist — and double the joist under every joint. A butt joint on a single joist gives each board about ¾ inch of bearing, which is not enough for either board.

Nearby sizes

Deck Cost by State — Priced From BLS Carpenter PayDeck quotes vary more within a town than between states, and this table shows why: carpenter pay ranges 45% across the country but only lands on 55% of the bill.
Deck Screws per Square Foot: The Count, the Length, and the Coating That MattersRoughly 350 screws per hundred square feet at standard spacing — and the coating rating matters more to the deck's life than the count does.
Cost to Replace Deck Boards Only: When Resurfacing Beats RebuildingResurfacing saves roughly half the cost of a rebuild, provided the frame underneath has more life left than the boards you are about to screw to it.
Deck Board Gap: Why Wet Lumber and Composite Need Opposite NumbersThere is no single correct gap. There is a correct gap for a board that is going to shrink and a different one for a board that is going to grow.