How-to · Updated October 2, 2026
How many Trex boards do you need for a deck?
Direct answer
A 20 × 12 ft deck at 16 inches on center needs 15 boards. With a 10% waste factor, order 17 boards.
The 20-foot example
20 ft × 12 = 240 inches. 240 ÷ 16 = 15. Ten percent waste adds 2 boards, and the order rounds to 17.
| Joist spacing | Boards | With 10% waste |
|---|---|---|
| 12 in | 20 | 22 |
| 16 in | 15 | 17 |
| 24 in | 10 | 11 |
Other deck sizes at 16 inches
| Deck | Area | Boards | Order with 10% waste |
|---|---|---|---|
| 12 × 10 ft | 120 sq ft | 9 | 10 |
| 16 × 12 ft | 192 sq ft | 12 | 14 |
| 20 × 12 ft | 240 sq ft | 15 | 17 |
| 24 × 16 ft | 384 sq ft | 18 | 20 |
Enhance boards at $55 each put the 20 × 12 order at $935 before installation.
Limits
- Planning count from this calculator: one board per joist space along the deck length. It does not lay out a 5.5-inch board width.
- Railings, stairs, picture framing, and breaker boards are extra.
- Confirm spacing on the product label. Some composite lines allow 24 inches on center; others stay at 16.
Use the calculator
Installed price for the same deck: Trex deck cost per square foot. Change the size in the Trex calculator.