Formula
Joists are spaced along the length. Boards cross the width:
joists = floor(length × 12 / spacing) + 1
Boards equal the width in inches divided by the board face, rounded up. Beams and footings are not included.
Assumptions
- Joists run across the width and are spaced along the length.
- The first and last joists are both counted.
- One board face covers the whole deck.
Worked example
A 16 by 12 foot deck with joists at 16 inches needs 13 joists.
Input definitions
- Spacing
- Inches on center from one joist to the next.
- Board face
- The exposed width of one deck board. A nominal 6-inch board is often 5.5 inches.
Limitations
This is not a span table. Joist size, beam size, and footing depth depend on the load and the code where you build.
Questions
Why 13 joists?
Sixteen feet is 192 inches. Divided by 16 inch spacing that is 12 spaces, and the closing joist makes 13.
How many boards is that?
Twelve feet is 144 inches. A 5.5 inch face needs 27 boards after rounding up.