How to use the decking calculator
- Length along the boards is the direction the boards run. On most attached decks that's parallel to the house.
- Width across the boards is the other direction. It sets how many rows of boards you need.
- Board width is the actual width: 5/4×6 and 2×6 boards are 5 1/2", and 2×4s are 3 1/2".
- Gap: 3/16" to 1/4" is common for wood, while composites usually specify a gap based on temperature. Follow the board maker.
- Board length: if the deck is longer than one board, rows get a butt joint, and joints must land on a joist. Choose board lengths that work out evenly with your deck when you can.
The math
Rows = deck width ÷ (board width + gap), rounded up. Boards per row = deck length ÷ board length, rounded up. Each row crosses every joist, so fasteners = rows × joists × fasteners per crossing.
Example: a 16' × 12' deck with 5 1/2" boards and a 3/16" gap needs 144 ÷ 5.6875 = 25.3, so 26 rows. With 16' boards that's 26 boards or 416 linear feet. At 16" O.C. there are 13 joists, so about 338 hidden clips or 676 face screws.
Before you buy
- Joist spacing: many composite and diagonal layouts need joists 12" O.C. Check the decking span rating, then size the joists with the deck joist span calculator.
- Butt joints: stagger them, and add a sister block or double joist where two boards meet if the maker calls for it.
- Picture framing: a border around the edge adds roughly one board length per side plus blocking.
FAQ
How many deck boards do I need for a 12x16 deck?
With 5 1/2" boards and a 3/16" gap, 26 rows cover 12'. If the boards run the 16' way and you buy 16' boards, that's 26 boards, or 416 linear feet.
How do I calculate linear feet of decking?
Find the number of rows (deck width ÷ board width plus gap, rounded up) and multiply by the deck length in feet.
How many screws do I need for decking?
Two screws at every joist for each board. A 16' × 12' deck with 26 rows over 13 joists needs about 676 screws, plus some extra.
What gap should I leave between deck boards?
Wood is often installed with a 1/8" to 1/4" gap, and wet pressure-treated boards tight because they shrink. Composites set the gap by temperature. Always follow the board maker's instructions.