Formula
The width is the side the joists are spaced across. The depth is the length of each joist. Deck boards run across the joists, so each board is one width long, and the stack of boards covers the depth. The gap sits between boards. The last board has no gap after it.
joists = ceil(width ÷ spacing) + 1
boards = ceil((depth + gap) ÷ (board width + gap))
fasteners = boards × joists × fasteners at each crossing
A 12 ft width at 16 in on center is 144 ÷ 16 = 9 spaces, so 10 joists, and the last bay is a full 16 in. A 10 ft width is 120 ÷ 16 = 7.5, which rounds up to 8 spaces and 9 joists. The last bay is 8 in. That 8 in is reported. It is not called acceptable.
A 16 ft depth with 5.5 in boards and a 1/8 in gap is (192 + 0.125) ÷ 5.625 = 34.16, which rounds up to 35 boards. Two fasteners at each of 10 crossings is 700. Ten percent waste is ceil(35 × 1.10) = 39 boards to order. The 4 spare boards are not in the fastener count. Joist stock is 10 × 16 ft = 160 ft of whatever length you are counting. It is not a 2×8 and it is not a beam.
There is no ledger, no beam, no blocking, and no picture-frame border in the count. IRC R507 and the American Wood Council’s DCA 6, Prescriptive Residential Wood Deck Construction Guide, are where joist span, beam size, and footing size are looked up. The local amendment wins when it is stricter. This page does not copy those tables and it does not say a layout passes.
Worked examples
12 ft wide, 16 ft deep, 5.5 in boards, 1/8 in gap, 16 in on center, two fasteners, 10% waste. 35 boards cover the deck, 39 with waste. 10 joists, last bay 16 in. 700 fasteners. 420 ft of decking installed, 468 ft ordered. 160 ft of joist stock.
The same deck at 12 in spacing. 144 ÷ 12 = 12 spaces, so 13 joists. Fasteners become 35 × 13 × 2 = 910. The board count does not change.
16 ft wide, no waste. 13 joists and a full 16 in last bay. Boards stay 35 if the depth stays 16 ft.
10 ft wide, 16 in spacing. 9 joists. The last bay is 8 in. That is a remainder, not a rating.
Reference
Worked counts for 5.5 in boards, a 1/8 in gap, two fasteners per crossing, and no waste. Depth is 16 ft except where noted. None of these rows is a joist size.
| Width | Spacing | Joists | Last bay | Boards | Fasteners |
|---|---|---|---|---|---|
| 12 ft | 16 in | 10 | 16 in | 35 | 700 |
| 12 ft | 12 in | 13 | 12 in | 35 | 910 |
| 16 ft | 16 in | 13 | 16 in | 35 | 910 |
| 10 ft | 16 in | 9 | 8 in | 35 | 630 |
Questions
Does this size the joists?
No. It counts joists, deck boards, and fasteners. It does not choose a 2×6, a 2×8, or a beam, and it does not say a span is allowed. IRC R507, AWC DCA 6, and the local building department decide that.
How are the boards counted?
Boards run across the joists. Each board is as long as the width. They are laid side by side across the depth, with the gap between boards and no gap after the last one. A 16 ft depth with 5.5 in boards and a 1/8 in gap needs 35 boards.
How many joists is that?
One joist at each end, and joists on the spacing in between. A 12 ft width at 16 in on center is 10 joists. A 16 ft width at 16 in is 13. When the width does not divide evenly, the last bay is shorter. That shorter bay is a count, not a pass.
Where do the fasteners go?
At each place a board crosses a joist. Two fasteners there, on the 12×16 ft example, is 700 fasteners for the 35 boards that cover the deck. Waste boards are extra for cuts. They are not given fasteners.
Sources
- International Code Council, IRC R507, exterior decks. Joist span, beam size, ledger attachment, and footings live there and in the local amendment. This page does not apply those tables.
- American Wood Council, DCA 6, Prescriptive Residential Wood Deck Construction Guide. The span tables and the connection details in that guide are the ones to use when the local code adopts it. This page counts pieces only.
- Board count is ceil((depth + gap) ÷ (board width + gap)), with the actual width you type. A nominal 6 in board is not assumed to be 5.5 in unless you enter 5.5. Joist count is ceil(width ÷ spacing) + 1, including both ends. A stair off the deck is the stair calculator. Roof geometry is the truss calculator. Wall studs are the framing calculator, which also stops at counts.