How to use this calculator
Two dimensions, and it matters which is which.
- Deck length is the direction the boards run. If you are buying 16 ft boards for a 16 ft deck, this is the 16.
- Deck width is what the joists cross. This is the span the boards sit across, and it determines how many rows of boards you need.
Then pick the board, the gap, the length your supplier sells, and the joist spacing. The spacing is not a free choice — see below.
The formula
The step people miss is that a board does not occupy its own width. It occupies its width plus one gap.
board pitch (in) = board width + gap
rows of boards = deck width (ft) × 12 ÷ board pitch, rounded up
boards = rows, if the board length matches the deck length
joists = deck length (ft) × 12 ÷ spacing, rounded down, + 1
screws = rows × joists × 2
Worked example — a 12 ft × 16 ft deck, 5/4 × 6 boards, ¼ in gap, 16 in joist spacing:
- Board pitch: 5.5 + 0.25 = 5.75 in
- Rows across the 12 ft width: 144 ÷ 5.75 = 25.04, so 26 rows
- Running 16 ft boards along the 16 ft length: 26 boards
- Joists across the 16 ft length: (192 ÷ 16) + 1 = 13 joists, each spanning 12 ft
- Screws: 26 × 13 × 2 = 676
That 25.04 rounding up to 26 is not a rounding error to shave off. The last row will be ripped narrower to fit, and you still have to buy a whole board to rip.
Board widths are not what they are called
A “2x6” is 1.5 by 5.5 inches. A “5/4 x 6” deck board is about 1 by 5.5 inches. Nominal sizes were the rough-sawn dimensions before the lumber was dried and planed, and the names stuck.
The calculator uses actual widths, which is why the options say “5.5 in actual”. If you are using a board not on the list, measure it — do not trust the label.
The gap between boards
A quarter of an inch is the usual figure for kiln-dried lumber and for composite decking.
Wet pressure-treated lumber is the exception. It arrives saturated and shrinks noticeably across its width as it dries, so those boards are often laid tight or with a very small gap, and open up to roughly the right spacing on their own over the first season. Lay wet lumber with a full quarter-inch gap and you will end up with half-inch gaps and a deck that drops phone screens through it.
The gap is not decorative. It lets water drain and air move under the boards. Decking laid tight and kept tight traps moisture against the joists and rots from underneath, which is the expensive kind of failure because the rot is in the frame rather than the surface.
Joist spacing is set by the decking, not by you
| Spacing | When |
|---|---|
| 16 in on centre | Standard for wood decking laid perpendicular to the joists |
| 12 in on centre | Most composite decking, and any decking laid diagonally |
| 24 in on centre | Only where the decking’s span rating explicitly allows it |
Composite boards are less stiff than wood and almost always specify 12 inches. Using 16 will produce a deck that feels springy between joists, and it voids the decking warranty — manufacturers are explicit about this.
Laying boards diagonally increases the unsupported span between joists by about 40 percent, so diagonal decking needs 12 in spacing too. Budget roughly 15 percent more decking for the angled cuts as well.
What this does not cover
This gives you decking, joists and fasteners. It deliberately stops there.
Beam size, post size, footing diameter and depth, the ledger connection to the house, joist hangers, and railing height and infill spacing are all governed by your local building code and by span tables for the specific species and grade you are buying. Most jurisdictions require a permit for an attached deck, and many require inspection of the footings before they are covered.
This is the part worth being blunt about: a deck that is short a few boards is an annoying second trip. A deck whose ledger is fastened to the house with the wrong hardware can come off the wall with people standing on it. The two are not the same kind of mistake, and a web page is the right tool for the first and the wrong tool for the second.
Ordering
Buy a couple of extra boards. Deck boards get cut wrong, arrive bowed, or turn out to have a knot exactly where a joist crossing needs a screw. Extras also let you cull the worst boards to the least visible edge.
If your deck is longer than the boards sold, plan the butt joints before you order: joints must land on a joist, and staggering them across the deck looks far better than lining them all up. That changes the board count, because every joint means two boards where one row previously needed one.
A material estimate, not a structural design. Confirm joist size, span and everything below the framing against your local code before you build.