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Deck Calculator: Boards, Joists and Screws
Work out how many deck boards, joists and screws you need for a new deck or a re-decking project, and what the boards will cost in wood or composite.
How many deck boards do I need?
Deck boards usually run parallel to the house, across the joists. The number of rows is the deck's depth in inches divided by the board width plus the gap. Multiply the rows by the deck length to get the total linear feet, divide by the board length, and add about 10% for cuts and bad boards.
Example: a 16 × 12 ft deck with 5.5 in boards and a 3/16 in gap has 144 ÷ 5.69 = 26 rows. Using 16 ft boards, that is 26 boards plus 10%, or 29 boards. Picking a board length that matches your deck avoids joints and waste.
Joist spacing
16 inches on center is standard for wood decking laid straight. Many composite brands require 12 inches for diagonal layouts or for stairs, and some thin boards can't span 16 inches. Always follow the decking maker's span rules, since the warranty depends on it.
Screws and fasteners
Face-screwing uses 2 screws per board at every joist. A 16 × 12 ft deck with 13 joists takes about 26 × 13 × 2 = 676 screws. Hidden clip systems use about the same number of clips as screw pairs. Use coated or stainless deck screws rated for treated lumber.
What this does not include
This calculator covers decking and joists. A full deck also needs a ledger, beams, posts, footings, joist hangers, flashing, stairs and railings, and usually a building permit. Your local building department can tell you the required footing size and depth.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
With 5.5 in boards and a 3/16 in gap, a 12 × 12 ft deck has 26 rows. Using 12 ft boards that is 26 boards, or 29 with 10% waste.
How far apart should deck joists be?
16 inches on center is standard. Many composite boards and diagonal layouts need 12 inches.