Deck board calculator
How many deck boards to buy, and roughly how many screws, for a new deck or a re-board.
How it works
Boards are laid side by side across the deck, so the number of rows is the deck width divided by one board plus one gap, rounded up (the last board can be ripped to fit). Each row needs one board if the deck is no longer than a board, or several joined end to end if it is. A short deck can get two or three rows out of each long board. We then add a few spare boards and count the joists under the boards to work out the screws: two screws wherever a board crosses a joist.
Worked example
A 12 × 10 ft deck with 5.5 in boards, a 3/16 in gap, 12 ft boards and joists at 16 in centres:
- Rows: 120 in ÷ (5.5 + 0.1875) in = 21.1, so 22 rows.
- Each row is one 12 ft board. With 5% spare, buy 24 boards.
- Joists: 144 in ÷ 16 in = 9 spaces, so 10 joists. Screws: 22 × 10 × 2 = 440.
A metric example: a 5.4 × 4 m deck in 140 mm boards with 5 mm gaps on joists at 450 mm takes 28 rows, so 30 boards of 5.4 m with 5% spare, and about 728 screws.
Choosing board length
Buy boards at least as long as the deck where you can: there are no joins to line up, and the deck looks cleaner. If the deck is longer than your longest board, every join has to land on a joist (many builders double the joist there), and joins should be staggered from row to row. If the deck is short, pick a board length that divides evenly, such as 12 ft boards for a 6 ft or 4 ft deck, so offcuts aren't wasted.
Gaps and joist spacing
The gap and the maximum joist spacing both depend on the board. Composite and PVC boards come with exact gap and span rules, and many need closer joists for diagonal layouts. Wood decking is often gapped at about 1/8 to 1/4 in (3–6 mm), depending on how dry it is when laid. Check the instructions for your boards, and check your local building rules for the frame itself.
Common questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and a 3/16 in gap, about 26 rows. If your boards are 12 ft long, that is 26 boards, plus a few spares.
How many screws per deck board?
Two at every joist. A 12 ft board over joists at 16 in centres crosses 10 joists, so it takes 20 screws.
Should deck boards run along or across the joists?
Across. Boards are fixed at right angles to the joists, or at an angle for diagonal decking, which usually needs closer joists.
Reviewed by Alex Rowan, Editor. Last reviewed . How we calculate · Report a problem