QuickCheckToolsDeck calculator
Work out how many deck boards, joists and screws you need for a rectangular deck.
What's included
Decking is assumed to run parallel to the house, with joists running out from the house at the spacing you choose. The count covers the field joists, deck boards and deck screws. Add the rim (band) joists, ledger board, beams, posts and footings, joist hangers and railings separately, as they depend on how your deck is supported. For post footings, use the concrete calculator.
Questions
How many deck boards do I need?
Divide the deck's depth in inches by the board width plus the gap to get the number of rows, then multiply by the boards needed per row. A 12 x 16 ft deck with 5½ in boards and a 3/16 in gap has 26 rows; with 16 ft boards that's 26 boards.
How many joists for a 12x16 deck?
At 16 in centres along a 16 ft width: 16 x 12 ÷ 16 = 12 spaces, so 13 joists, each 12 ft long. That's the field joists only; add the rim joists.
What joist spacing should I use?
16 in on centre is standard for most decking. Composite boards often need 12 in, and boards laid diagonally need closer spacing. Check the decking manufacturer's instructions.
How many screws per deck board?
Two screws at every joist the board crosses. A 12 x 16 ft deck with 13 joists and 26 rows needs about 676 screws. Hidden fastening systems have their own counts.
What gap should I leave between deck boards?
About 3/16 in (5 mm) is common for wood, so water drains and the boards can expand. Composite manufacturers specify their own gaps.
Does this tell me what size joists to use?
No. Joist and beam sizes depend on span, spacing, wood species and loads, set by the span tables in your local building code. This calculator counts materials only.
Materials estimate only. Joist, beam and footing sizes must follow the span tables and requirements of your local building code.