How to calculate deck boards
- Rows of boards: deck width ÷ (board width + gap), rounded up. A “6-inch” board is really 5.5 in wide.
- Boards per row: deck length ÷ board length, rounded up. If the deck is longer than your boards, each row has a joint.
- Total: rows × boards per row, plus about 5% for split or bowed boards.
Worked example: a 12 × 16 ft deck with 16 ft boards
Each board plus a 3/16 in gap covers 5.69 in. 144 in ÷ 5.69 = 25.3, so 26 rows. One 16 ft board covers each row: 26 boards, plus 5% = 28. With joists every 16 in there are 13 joists, and 2 screws per board at each joist makes 676 screws.
Choosing board length
Match the board length to the deck where you can. A 16 ft deck built from 16 ft boards has no joints. If the deck is longer, joints must land on a joist and should be staggered so they don’t line up row after row. Many builders add a second joist (sister) at butt joints so each board end has full support.
Joists and spacing
| Decking | Typical joist spacing |
|---|---|
| Wood boards, laid straight | 16 in (400 mm) |
| Composite or PVC, laid straight | 12–16 in, per the manufacturer |
| Any board laid diagonally | Often 12 in (300 mm) |
Joist size and span depend on the lumber and your local code. This calculator counts joists only to work out fasteners; size the framing from your local span tables.
Footings for the deck posts can be estimated with the concrete calculator using the round column option.
Gaps and fasteners
Composite makers specify the gap between boards, often around 3/16 in. Wet pressure-treated wood shrinks as it dries, so it is often laid tight or with a small gap. Use two screws per board at each joist, or one hidden clip per crossing if you use a hidden fastening system.