How many deck boards do I need?

Nominal size is not actual size

Decking is counted by rows, not by area. Because boards run the full length of the deck, the number of rows is set entirely by the width: divide the width in inches by the board width plus the gap, and round up. A 12 foot wide deck with 5.5 inch boards and a quarter inch gap needs 26 rows. Multiply rows by the deck length and you have your linear feet.

The trap is board width. A 2x6 is not 6 inches wide — it measures 5.5 inches, because nominal lumber sizes describe the rough-sawn dimension before drying and planing. A 2x4 is 3.5 inches, a 2x8 is 7.25. Using the nominal number instead of the actual one will leave you roughly 8% short, which on a large deck is several hundred dollars of missing boards. Composite decking does not follow the same convention at all, so measure a sample.

Gaps are not optional. Pressure-treated lumber arrives wet and shrinks as it dries, so it is often laid tight and opens up on its own. Kiln-dried and composite boards do not shrink much and need a deliberate quarter inch gap for drainage and expansion. Without it, water sits on the deck and the boards cup.

Two things this count does not include: the framing underneath, and the fact that every butt joint must land on a joist. If your deck is longer than the boards you can buy, you are not just adding boards — you are planning where the seams fall, and usually adding blocking or a doubled joist to land them on.