How many deck boards for a 12x16 deck?

Rows, not square feet

Decking is sold as boards of a fixed length, so the useful answer is "how many 16 ft boards", not "how many square feet". The row count is what drives it:

rows = deck width in inches ÷ (board width + gap)

A "5/4x6" deck board is 5-1/2 in wide, not 6. That is the minimum dressed dry width for a nominal 6 in board in the American Softwood Lumber Standard (PS 20-20, Table 3); the 5/4 refers to the nominal 1-1/4 in thickness, which dresses to a full 1 in. A 2x6 is the same 5-1/2 in wide.

For a 12 ft × 16 ft deck with boards running the 16 ft way and a 3/16 in gap: 144 ÷ (5.5 + 0.1875) = 25.3, so 26 rows of 16 ft board. Run the same calculation on a nominal 6 in and you get 24 rows — two boards short, leaving about 11 in of open joist along the last edge.

The gap depends on what you bought

Trex publishes 3/16 in as the width-to-width gap for composite decking, and most composite makers are in the same range; the gap is a drainage and expansion requirement, not a style choice, so it is worth honouring.

Wet pressure-treated southern pine is the opposite case. It leaves the treating cylinder at roughly 25% moisture content, and it shrinks across the width as it dries. Install those boards butted tight and they open to something close to 1/8 in on their own. Gap them at 3/16 as well and you end up with 3/8 in slots that drop tools and phone cases. Kiln-dried-after-treatment (KDAT) boards have already shrunk, so they get the gap.

Pick the stock length before you buy

Stock lengths are 8, 10, 12 and 16 ft, and 20 ft is usually a special order. Two rules decide the cheapest buy:

• Any butt joint has to land on a joist, and the two boards each need their own fasteners into it. That is why a 16 ft board across a 16 ft deck — no joints at all — is worth paying for.
• Price per foot is not flat. Ten-foot boards are often dearer per foot than twelve, so a 10 ft deck can be cheaper to build from 12 ft stock cut down. The offcut figure above tells you how much you are throwing away per row.

Screws come from joist crossings

Two screws per board at every joist, so rows × joists × 2. That 12×16 deck has 13 joists at 16 in on centre, giving 26 × 13 × 2 = 676 screws. Rules of thumb that quote screws per square foot miss the fact that tightening the joist spacing to 12 in raises the count by a third without changing the deck area.

Buy them by piece count, not by weight. Deck screws are shelved in pound boxes, but the count in a 5 lb box swings with gauge and length, so the only reliable number is the one printed on the box. Enter that above and the box count follows.