Plan the materials for a rectangular deck. Enter the deck size, the board width and gap, and the length of the boards you will buy to get the number of deck boards, joists and screws.
Boards run along the deck length and sit on joists that run across them. Joist sizes and beam spans depend on loads and local building codes, so check those separately.
Formula
Rows = ⌈Deck width ÷ (Board width + Gap)⌉ · Boards = Rows × ⌈Deck length ÷ Board length⌉
Joists = ⌈Deck length ÷ Joist spacing⌉ + 1, and screws ≈ 2 per board per joist.
Worked example
A 16 × 12 ft deck with 5½-inch boards and ⅛-inch gaps needs ⌈144 ÷ 5.625⌉ = 26 rows of 16 ft boards — 29 boards with 10% extra. At 16-inch joist spacing there are 13 joists, and about 676 screws.
Frequently asked questions
What gap should I leave between deck boards?
Typically ⅛ to ¼ inch (3–6 mm) for wood and whatever the manufacturer specifies for composite. Wet pressure-treated boards can be laid tight because they shrink as they dry.
What joist spacing do deck boards need?
16 inches on centre is standard for most wood and composite decking; diagonal layouts and some composites need 12 inches.
Last updated · Calculations checked by automated tests· Editorial policy