How decking is calculated
Rows of boards = ⌈deck width × 12 ÷ (board width + gap)⌉
Linear feet = rows × deck length
Boards = linear feet × (1 + waste %) ÷ board length, rounded up
Joists = ⌊deck length × 12 ÷ spacing⌋ + 1
Face screws = rows × joists × 2
Two face screws per board at every joist is the usual fastening pattern; hidden-fastener systems have their own counts. Joist spacing must not exceed what your decking manufacturer allows — composite boards often require closer spacing than wood, especially on diagonal layouts.
Worked example
A 16 × 12 ft deck with 5.5 in boards, 3/16 in gaps, 16 ft boards, joists 16 in on center, 10% waste:
| Rows (144 ÷ 5.6875) | 26 |
|---|---|
| Linear feet | 416 |
| 16 ft boards | 29 |
| Joists | 13 |
| Face screws | 676 |
Framing (beams, posts, ledger, hardware) and railings are not included. Decks are structural: follow local code and get permits.
Frequently asked questions
What gap should I leave between deck boards?
It depends on the material and moisture content. Composite makers specify a gap in their installation guide; wet pressure-treated boards are often laid tight because they shrink as they dry.
Should boards run the long way?
Usually boards run parallel to the house and across the joists. Choose a board length that divides the deck length evenly to reduce waste and joints.