How to calculate deck boards
- Rows. Divide the deck width by one board plus one gap, and round up.
- Total length. Multiply the rows by the deck length and add about 10 percent for cuts and defects.
- Boards. Divide by the board length and round up.
rows = width ÷ (board width + gap) · boards = rows × length × 1.10 ÷ board length
Worked example
A 16 × 12 ft deck with 5.5 in boards and 3/16 in gaps: 144 in ÷ 5.6875 in = 25.3, so 26 rows. 26 × 16 ft = 416 linear ft, or 458 with waste. With 16 ft boards, buy 29.
Board widths
Lumber is sold by nominal size, which is larger than the real size. A "6 inch" deck board is about 5.5 in wide, and a nominal 2×6 is 1.5 × 5.5 in. In Australia, deck boards are sold by actual width, commonly 86, 90, 140 or 145 mm. Composite boards have their own sizes, so read the product sheet.
Gaps
Gaps let water drain and leaves fall through. A gap of about 1/8 to 1/4 in (3 to 6 mm) is typical once boards have dried. Wet pressure-treated lumber shrinks, so it is often laid tight and opens up as it dries. Composite and PVC boards come with specific spacing rules, often with hidden fasteners that set the gap.
Joists and fasteners
Joists at 16 in (about 400 to 450 mm) centres are common for wood decking laid square to the joists. Diagonal layouts and some composite boards need 12 in spacing. The usual pattern is two screws where each board crosses each joist. Joist size, span, beams and footings depend on loads and local code; get those designed or checked before you build.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and a 3/16 in gap, a 12 ft wide deck needs 26 rows. With 12 ft boards and 10 percent waste, buy about 29 boards.
How many screws per deck board?
Two at every joist. A 16 ft board on joists 16 in apart crosses 13 joists, so it takes about 26 screws.
Should deck boards be gapped?
Yes, in most cases. Gaps of about 1/8 to 1/4 in let water drain. Follow the manufacturer's spacing for composite boards.
Last updated November 1, 2026.