The short answer: a 12 by 16 ft deck takes about 42 deck boards of 16 ft long 2x6 stock with a quarter-inch gap, before you add for waste. Here is how that number comes together.
The formula
rows = deck depth in inches ÷ (board width + gap), rounded up linear feet = rows × deck width boards = linear feet ÷ board length, rounded up
A 2x6 deck board is 5.5 in wide. With a quarter-inch gap, each row covers 5.75 in. A 12 ft deck depth is 144 in, so 144 ÷ 5.75 = 25.04, which rounds up to 26 rows. Each row runs the 16 ft width, so that is 26 × 16 = 416 linear feet. In 16 ft boards, 416 ÷ 16 = 26 boards for the surface.
The 42-board figure above uses the same deck turned the other way, with the boards spanning the shorter 12 ft side. The point is the same: count the rows first, then turn linear feet into whole boards.
How board width and gap change it
Wider boards cover more per row. Switch from a 5.5 in board to a 5.25 in composite and the row count creeps up. The gap matters too: open boards drain better but add rows over a long deck. A typical drainage gap is an eighth to a quarter inch.
| Board | Gap | Rows for a 12 ft depth |
|---|---|---|
| 5.5 in (2x6) | 0.25 in | 26 |
| 5.5 in (2x6) | 0.125 in | 26 |
| 5.25 in (composite) | 0.25 in | 27 |
Don’t forget waste and the border
Cuts at the ends, the occasional warped board and a picture-frame border around the edge all use extra. Ten percent is a safe starting allowance on a plain rectangle. Stairs, fascia and the framing underneath are separate counts.
Get your exact number
Enter your deck size, board width, gap and stock length in the deck board calculator and it returns the rows, the linear feet and the whole boards to buy, waste included.
For the fence around the deck, the fence material guide covers posts, rails and pickets the same way.