What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Deck length along the boards: 16 ft. The dimension parallel to the deck boards.
- Deck width across the boards: 12 ft.
- Board width (actual): 5.5 in. A nominal 5/4×6 or 2×6 board is 5.5 in wide.
- Gap between boards: 0.1875 in. Follow the board manufacturer. 3/16 in is common for composite.
- Board length: 16 ft.
- Joist spacing: 16 in.
- Extra for waste and cuts: 10 %.
What happens, step by step
- Joists go in first, one every 16 in.
- Boards go across the joists row by row, 3/16 in apart.
- Two screws go in wherever a board crosses a joist.
- 26 rows need 29 boards, with 10% extra for cuts.
The answer: boards to buy is 29 boards.
How it’s calculated
- Rows = (deck width + gap) ÷ (board width + gap), rounded up
- Boards per row = deck length ÷ board length, rounded up
- Boards = rows × boards per row × (1 + waste %)
- Joists crossed = deck length ÷ joist spacing + 1; screws = rows × joists × 2
Worked example
16 × 12 ft deck, 5.5 in boards with 3/16 in gap, 16 ft boards, joists 16 in on center, 10% waste.
Inputs
- Deck length along the boards
- 16 ft
- Deck width across the boards
- 12 ft
- Board width (actual)
- 5.5 in
- Gap between boards
- 0.1875 in
- Board length
- 16 ft
- Joist spacing
- 16 in
- Extra for waste and cuts
- 10 %
- Price per board
- 0 $
Results
- Boards to buy
- 29 boards
- Rows of boards
- 26
- Total linear length
- 416 ft
- Face screws (2 per joist crossing)
- 676
Assumptions and limits
- Rectangular deck with boards running parallel to one side (no diagonal or picture-frame layout).
- Off-cuts are not reused between rows.
- Hidden-fastener systems use their own clip counts — check the manufacturer.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How many deck boards do I need for a 12×16 deck?
With 5.5 in boards and a 3/16 in gap, a 12 ft width needs 26 rows. Running 16 ft boards, that is 26 boards, or 29 with 10% waste.
How much waste should I allow for decking?
5–10% is typical for straight layouts. Diagonal and herringbone patterns commonly need 15% or more.
How far apart should deck boards be?
Follow the manufacturer. Composite boards often need about 3/16 in; wet pressure-treated boards are often laid tight because they shrink as they dry.