Calculate exactly how many deck boards, joists, posts, and screws you need for your deck project. Supports standard lumber and composite decking with adjustable board sizes, joist spacing, and waste factors.
Joist span direction
Board direction
Building a 12×16 foot deck with 5/4×6 boards at 12-foot lengths, 16-inch joist spacing, and 10% waste? You need approximately 58 deck boards, 13 joists, 4 rim joists, 8 post footings, and about 7 boxes of deck screws (676 screws). At $8–$12 per pressure-treated board, the decking alone runs $464–$696.
| Deck Size | 5/4×6 Boards | Joists (16" OC) |
|---|---|---|
| 10×10 ft | 25 | 9 |
| 10×12 ft | 25 | 10 |
| 12×16 ft | 58 | 13 |
| 14×20 ft | 66 | 16 |
| 16×24 ft | 75 | 19 |
Board counts include 10% waste factor.
Divide your deck width by the board width (including gap), then multiply by the number of boards needed along the length. Add 10% for waste from cuts and defects.
16 inches on-center is standard for most wood decking. Use 12 inches for composite decking, diagonal board patterns, or spans over 12 feet. 24-inch spacing is only suitable for 2×6 boards on short spans.
3/16 inch is standard for pressure-treated lumber (it shrinks as it dries). Use 1/8 inch for pre-dried wood and composite boards that do not shrink.
Two screws at every joist crossing. For a 12-foot board spanning joists at 16-inch spacing, that is approximately 18 screws per board.
Below your local frost line, which varies by region. Typically 36–48 inches in northern climates, 12–24 inches in southern climates. Check your local building code.
Wondering what a deck actually costs? See our full deck cost guide for 2026 pricing by material and size.
Typical Results
💡 If board count seems low, check you haven't entered joist spacing in feet instead of inches.