Square footage is the wrong number
Every decking calculator will tell you a 12×12 deck is 144 square feet. That's true and it doesn't help, because you don't buy square feet — you buy boards, and the count depends on how wide each board actually covers once it's installed.
One number does all the work: 0.469 feet
A nominal 5/4×6 deck board measures 5.5 inches across its face. A 2×6 measures the same 5.5 inches. Add the standard 1/8-inch expansion gap and each installed row covers 5.625 inches — which is 0.469 feet.
rows of boards = deck span ÷ 0.469 ft
A 12 ft span ÷ 0.469 = 25.6 → 26 rows. Always round up: the last row is a real row.
From there it's arithmetic. Multiply rows by the board run length to get linear feet, add 10% for cuts and defects, then divide by the board length you're buying.
Chart: decks by size
Assumes 5/4×6 or 2×6 boards, a 1/8-inch gap, 10% waste, and 16-inch on-centre joists.
| Deck size | Rows | Linear ft (+10%) | 12 ft boards | 16 ft boards |
|---|---|---|---|---|
| 10 × 10 | 22 | 242 | 21 | 16 |
| 10 × 12 | 26 | 286 | 24 | 18 |
| 12 × 12 | 26 | 344 | 29 | 22 |
| 12 × 16 | 35 | 462 | 39 | 29 |
| 12 × 20 | 43 | 568 | 48 | 36 |
| 12 × 24 | 52 | 687 | 58 | 43 |
| 14 × 14 | 30 | 462 | 39 | 29 |
| 16 × 16 | 35 | 616 | 52 | 39 |
| 20 × 20 | 43 | 946 | 79 | 60 |
Board counts are driven by deck area, so which way the boards run changes the total by at most one board. The lengths you cut do change a lot — see below.
Board length matters as much as board count
Look at the 12×12 row: 344 linear feet either way, but 29 boards of 12-foot stock versus 22 boards of 16-foot stock. That looks like the 16-foot option wins on price per board. It usually doesn't.
On a 12×12 deck, 16-foot boards mean squaring off and cutting 4 feet from every single board. You paid for 16 feet and installed 12, which makes the "cheaper per foot" board the more expensive deck. For square decks, match the board length to the span that runs across the joists and the waste mostly disappears. That's why 10-foot stock is the right buy for a 10×10 even though it costs more per linear foot.
Why 10% waste and not 15%
A straight-lay deck with boards running one direction is the least wasteful layout in carpentry. Ten percent covers the cuts at the ends, the occasional split board, and the one row you measure twice. Step up to 15% for a diagonal layout, and treat a herringbone or picture-frame border as its own project — diagonals throw away real material, because every board that hits the rim joist at 45° produces an offcut too short to reuse.
Joists and screws
Joist count is set by the board run direction: joists sit perpendicular to the boards, so a 12-foot run needs 10 joists at 16 inches on centre, and a 20-foot run needs 16. If you're installing composite or any rigid board, use 12-inch spacing — composites span less than wood and the manufacturers' warranties are written around it.
Fasteners run about 4 screws per square foot on 16-inch joists, which puts a 12×12 deck at 572 screws and a 16×16 at 1,001. The ratio is not fixed: a 10×10 runs 4.4 per square foot and a 20×20 runs 3.8, because a longer run needs proportionally fewer joists to cross. Buy by the box.
Still deciding what to buy? The 5/4×6 vs 2×6 comparison covers the actual dimensions, span limits, stiffness math, and boards per 100 square feet for each profile.
Run your own deck
The deck calculator takes your exact dimensions, board profile, board length, and joist spacing — then returns boards, linear feet, joists, and screws in one pass. That link opens on the 12 × 12 deck from the chart above: 26 rows and 344 linear feet before it turns those into lumber. Change the dimensions to your real measurements before you drive to the yard; the whole point is that a 12×13 deck is not a 12×12.