Deck Board Calculator
Deck boards, joists and fasteners, with the installation gap that differs between wet treated lumber and pre-dried or composite boards.
Board count, fasteners and the gap. Wet treated lumber shrinks as it dries and kiln-dried does not, so the right gap at installation is different for each — and getting it backwards shows.
Quick answer: deck boards and joists
| Length × width | Rows | Boards +10 % | Joists at 16 in. |
|---|---|---|---|
| 10 × 10 ft | 22 | 25 | 8 |
| 12 × 12 ft | 27 | 30 | 10 |
| 16 × 12 ft | 27 | 30 | 13 |
| 20 × 12 ft | 27 | 30 | 16 |
| 20 × 16 ft | 35 | 39 | 16 |
| 24 × 16 ft | 35 | 39 | 19 |
5-1/2 in. wet treated boards butted tight, each running the deck's length; joists at 16 in. on center. Kiln-dried or composite boards need a gap from the start, so each row covers more and the count drops.
| Deck area | — |
|---|---|
| Coverage per board | — |
| Rows of decking | — |
| Boards (with waste) | — |
| Linear feet | — |
| Joists | — |
| Fasteners (approx) | — |
The gap is not a setting. It is the difference between the board’s width now and its width later. Pressure-treated Southern Pine leaves the treating cylinder soaked, and the Southern Forest Products Association’s deck guide says only to adjust spacing between boards during installation to compensate for shrinkage as the wood dries. It doesn’t give you a number. The Wood Handbook does. Table 4‑3 of FPL‑GTR‑190 puts tangential shrinkage, green to ovendry, at 7.4% for loblolly through 7.7% for shortleaf, the spread across the four Southern Pine species. Equation 4‑7 prorates that from a 30% fibre saturation point, and Chapter 13 puts thoroughly air-dried lumber at 12% to 15%. So 7.4 × (30 - 15) / 30 = 3.7%, and 7.7 × (30 - 12) / 30 = 4.6%. On a 5‑1/2″ flat-sawn board that is 0.20″ to 0.25″ of width, and the board opens that much on its own as it dries.
So count the gap you install, not the gap you end up with. A 12 ft width in 5‑1/2″ boards butted tight takes 27 rows. The same width with 1/4″ written into it takes 26. Set the gap to zero for wet stock and let it dry into the spacing. Boards kiln-dried after treatment are the exception. Those hold their width, so the gap you type is the gap you keep.
Composite has the opposite problem. Nothing opens on its own; the movement is thermal and lengthwise. The side gap is a real input. TimberTech’s composite guide asks 1/8″ minimum to 3/16″ maximum, and it stays where you put it. The butt joint does not: 3/16″ at 32°F and below, 1/8″ from 33°F to 74°F, 1/32″ at 75°F and above. That is a six-fold range on one joint, so check the temperature on the day you fasten and set the butt gaps to it.
What I check on a job
What I check before I order. Before I order from this count, I check which way the boards run. The page lays every row the full deck length and counts joists along that same length, so length has to be the dimension the boards run, not the longer side. Then I check the joist count against the end of the deck. It stops at the last full spacing: a 15 ft deck at 16 inches gets 12 joists, and the last one sits 4 inches short of the end, so I add the end joist by hand. I also look at the last row. Rows are rounded up, and 12 ft of tight 5-1/2 inch boards is 26 full rows plus a strip about an inch wide. I would rather adjust the width or edge detail than install a sliver. When the deck is longer than the boards I can get, each row becomes two or more boards, and I lay out where those butt joints fall against the joists before I trust the board count.
Rows are deck width ÷ (board width + gap), rounded up. Fasteners assume two per
board per joist crossing, which is the ordinary pattern for face-screwed decking; hidden fastener
systems have their own counts.
The gap is the part that goes wrong. Pressure-treated lumber is usually delivered wet. It shrinks
across its width as it dries, so it is normally installed butted tight and opens up on its own. Kiln-dried
wood and composite are already at or near their final width, so the gap has to be built in at
installation or the deck will not drain. Follow the manufacturer's instruction for the specific board.
Composite in particular has required gaps that also change with the temperature on the day.
Worked example
A 16 × 12 ft deck in 5-1/2 in boards, joists at 16": how many boards and joists?
- Rows across 12 ft: ceil(144 ÷ 5.5) = 27 rows of 16 ft boards.
- 27 × 16 = 432 lineal ft; at 16 ft a board that is 27 boards before waste.
- Joists at 16" over 16 ft: 16 × 12 ÷ 16 + 1 = 13.
27 rows, 432 lineal feet of decking, 13 joists, and the fasteners per board-crossing. Wet treated lumber is laid tight; dry or composite boards get a gap, and the gap changes the row count.
8 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How many deck boards for a 16 × 12 ft deck?
27 rows of 16 ft boards at 5½ inches wide, 432 lineal feet, before waste.
Should I leave a gap between deck boards?
Wet treated lumber is laid tight and shrinks to its gap; kiln-dried and composite boards need a gap at installation, which changes the row count.
How much does pressure-treated pine shrink as it dries?
3.7% to 4.6% of its width, from the Wood Handbook (FPL-GTR-190, Table 4-3 and Equation 4-7): 0.20 to 0.25 in. on a 5-1/2 in. board. That is why wet stock goes in butted tight and opens on its own; set the gap to zero for it. Kiln-dried-after-treatment boards keep their width.
What gaps does composite decking need?
TimberTech asks 1/8 in. minimum to 3/16 in. maximum at the sides, which stays put. Butt joints follow the day's temperature: 3/16 in. at 32 °F and below, 1/8 in. from 33 to 74 °F, 1/32 in. at 75 °F and above. Check the temperature on the day you fasten.