Board count for a 12×20 deck by board length
5.5-inch boards with 1/8-inch gaps, 10% waste.
| Board length | Boards to buy | Total linear ft | Butt joints? |
|---|---|---|---|
| 12 ft | 48 | 576 ft | Yes — stagger joints on joists |
| 16 ft | 36 | 576 ft | Yes — stagger joints on joists |
| 20 ft | 29 | 580 ft | None |
The highlighted length wastes the least lumber for a 20 ft run. Rows need more than one board, so stagger the joints and land every joint on a joist.
Joists and screws
- At 16 in on center: 16 joists spanning 12 ft, about 832 screws (3 boxes of 350)
- At 12 in on center (composite or diagonal): 21 joists and about 1,092 screws
Joist size depends on the span, wood species and local code. A 12 ft span is a common size, but confirm joist dimensions with your local building department's span tables.
The math
12 ft × 12 ÷ (5.5 + 0.125) = 25.6 → 26 rows
26 rows × 20 ft = 520 linear ft
520 × 1.10 ÷ 12 ft = 48 boards
Common questions
How many deck boards for a 12x20 deck?
48 boards of 12 ft (5.5-inch width, 10% waste included).
How many joists for a 12x20 deck?
16 joists at 16 inches on center, or 21 at 12 inches.
How many screws for a 12x20 deck?
About 832 deck screws — two per board at every joist.
Want the full method, tips and other sizes? See the deck board calculator guide.