A 16 × 12 ft (4.88 × 3.66 m) deck needs 13 joists at 16 in (406 mm) centres and 26 rows of boards: 29 boards of 16 ft with 10% waste, held by about 676 screws.
Deck plan: joists dashed, boards across
How it's calculated S
Joists
⌈16in192in⌉+1=13joists of12ft
Rounded up, plus the joist at the far end. Rim joists and beams are extra.
Rows of boards
⌈5.5+0.125144in⌉=26
Rounded up; the last row may need ripping.
Boards
⌈1626×16×1.1⌉=29
Rounded up to whole boards.
Fasteners
26×13×2=676
Rule of thumb: two face screws, or one hidden clip, where each board crosses each joist.
About the deck calculator
Boards run along the house and joists run out from it. The joist count is the deck width divided by the joist spacing, rounded up, plus one for the far end. Rows of boards are the deck depth divided by board width plus gap, rounded up. Boards to buy are rows × deck width × (1 + waste) ÷ board length, rounded up, and fasteners are counted where each row crosses each joist.
The default 16 × 12 ft deck, with joists 16 in on centre and 5½ in boards gapped ⅛ in, needs 13 joists and 26 rows: 416 linear feet, or 29 sixteen-foot boards with 10% waste, held by 676 face screws.
Rim joists, beams, posts, ledger, blocking and stair stringers are not counted. Joist size and span depend on species, grade and load; take them from the span tables in 2021 or 2024 IRC section R507 or from AWC DCA 6.
How to calculate deck boards and joists
Joists are width ÷ joist spacing plus one, and board rows are depth ÷ (board width + gap), both rounded up; boards are rows × width plus waste, ÷ board length. A 12 × 12 ft deck at 16 in centers with 5½ in boards and ⅛ in gaps needs 10 joists, 26 rows and 29 boards 12 ft long.
Deck materials step by step
Width runs along the house, the way the boards run. Depth runs out from the house, the way the joists run. For a 16 × 12 ft deck:
Joists: 16 ft is 192 in, and 192 ÷ 16 in = 12 spaces, so 13 joists including the far end.
Rows: 12 ft is 144 in, and 144 ÷ (5.5 + 0.125 in) = 25.6, so 26 rows.
Decking length: 26 rows × 16 ft = 416 linear feet.
Boards: 416 × 1.10 ÷ 16 ft = 28.6, so 29 boards 16 ft long.
Fasteners: 26 rows × 13 joists × 2 = 676 face screws, or 338 hidden clips at one per crossing.
Joist spacing for deck boards
2021 and 2024 IRC Table R507.7 allows joists 16 in apart under 5/4 wood boards laid square to them, and 12 in under diagonal boards, when each board crosses at least three joists.
2x wood boards can span 24 in, the most the table allows for any decking, and composite boards use the span on their ASTM D7032 label.
Closer joists add crossings: the 16 × 12 ft deck at 12 in centers has 17 joists and 884 face screws.
Joist size and span depend on species, grade and load, so take them from the IRC section R507 span tables or the American Wood Council DCA 6 guide. Rim joists, beams, posts, the ledger, blocking and stair stringers are separate items.
Board length and the cut plan
The board count adds up total length, so it assumes offcuts get used. With 20 ft boards and no waste, the 416 ft of decking is 21 boards, but each 16 ft row needs its own unspliced board, so the deck takes 26.
Boards shorter than the deck is wide put a butt joint in every row. Stagger the joints and land every end on a joist, adding blocking or a doubled joist where needed. With 12 ft boards on the 16 ft deck, the count rises to 39 boards with 10% waste.
Boards, joists and fasteners for five decks
"Default" is the 16 × 12 ft deck worked above. "Hidden clips" changes the fastening, "Twelve inch centres" the joist spacing, and "No waste" drops the 10% allowance.
"Metric deck" is 4.8 × 3.6 m with joists 400 mm apart, 140 mm boards with 5 mm gaps, 4.8 m boards and hidden clips.
Scenario
Aggregate boards
Joists
Screws or clips
Default
29.000000
13.000000
676.000000
Hidden clips
29.000000
13.000000
338.000000
Twelve inch centres
29.000000
17.000000
884.000000
No waste
26.000000
13.000000
676.000000
Metric deck
28.000000
13.000000
325.000000
Changing the fastening or the joist spacing leaves the boards alone, and changing the waste leaves the joists and fasteners alone.
Mistakes that leave a deck short
Using the nominal board width. A nominal 6 in deck board is 5½ in wide. Using 6 in gives 24 rows and 27 boards for the 16 × 12 ft deck instead of 26 rows and 29 boards.
Forgetting the end joist. 192 in at 16 in centers is 12 spaces, which need 13 joists.
Counting long boards by total length. As above, 20 ft stock on a 16 ft deck still needs one board per row unless the offcuts can be spliced in.
Using square-deck waste for diagonal boards. Diagonal decking cuts more boards, so allow 15% or more, and it needs joists closer together.
Inputs worth a second look
Board width is the actual covering width; the gap is ⅛ in for wood and commonly 3/16 in for composite.
Face screws count two per crossing of a board and a joist; hidden clips count one.
16 in on centre suits 5/4 wood boards laid square to the joists. 2021 and 2024 IRC Table R507.7 allows 5/4 boards 16 in when each board crosses three or more joists (12 in over two), and 12 in when laid diagonally (8 in over two). 2x boards may span 24 in, except 18 in for diagonal boards over two joists. Composite boards use the span on their ASTM D7032 label.
How many deck boards do I need?
Divide the deck depth by the board width plus gap to get rows, then multiply by the deck width and add waste. For a 16 × 12 ft deck with 5½ in boards and ⅛ in gaps: 144 ÷ 5.625 = 25.6, so 26 rows; 26 × 16 ft = 416 linear feet; with 10% waste, 457.6 ft, or 29 boards 16 ft long.
How many screws do I need for a deck?
Two face screws wherever a board crosses a joist. A 16 × 12 ft deck at 16 in centres has 13 joists and 26 rows, so 26 × 13 × 2 = 676 screws, about 3.5 per square foot. Hidden clips take one per crossing, 338 here. Closer joist spacing adds crossings: at 12 in centres the same deck needs 884 screws.
How many joists do I need for a deck?
Divide the width along the house by the joist spacing, round up and add one. A 16 ft (192 in) deck at 16 in centres needs 192 ÷ 16 + 1 = 13 joists; at 12 in centres it needs 17, and at 24 in, 9. Rim joists, doubled joists under board splices and the beam are extra.
International Code Council — 2021 and 2024 IRC section R507 (exterior decks), Table R507.7 maximum joist spacing for decking
Checked against references
This calculator includes 3 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.
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