To find how many deck boards you need, divide the deck width in inches by the board width plus the gap to get the number of rows, multiply rows by the deck length for linear feet, then divide by your board length and add about 10 percent. With the deck calculator's defaults (5.5 in boards and a 1/8 in gap), a 16 × 12 ft deck takes 26 rows, or 416 linear feet. That's 29 boards of 16 ft with 10 percent extra.
Rule of thumb for 6 inch nominal decking: plan on about 2.1 rows per foot of deck width. The calculator below also estimates the number of joists.
How to measure your deck
- Decide which way the boards run. Boards usually run across the joists. The calculator's "deck length" is the direction the boards run, and "deck width" is the direction you count rows. The American Wood Council's DCA 6 guide allows decking from perpendicular to the joists up to a 45 degree angle.
- Measure the length along the boards. Include any overhang past the rim joist or fascia you plan to leave.
- Measure the width across the boards. This is the dimension that sets the number of rows.
- Use the board's actual width. Trex's 1 × 6 profile boards are 5.5 in wide. A nominal 2 × 6 is also 5 1/2 in wide, because the Southern Pine Inspection Bureau notes that 2 × 4s and 2 × 6s are 1/2 inch narrower than their nominal width. Check the label on anything else.
- Pick the gap. DCA 6 calls for about 1/8 inch between boards, which is the calculator's default of 0.125 in. Trex requires at least 3/16 inch side to side for its composite boards, so enter 0.1875 for Trex.
- Pick a board length that fits. Trex boards come in 12, 16, and 20 ft lengths. Choose the one that divides most evenly into your deck length (more on that below).
The deck board formula
The calculator uses:
- Rows = deck width (ft) × 12 ÷ (board width + gap), rounded up
- Linear feet = rows × deck length (ft)
- Boards = linear feet × (1 + extra % ÷ 100) ÷ board length, rounded up
- Joists = deck length (ft) × 12 ÷ joist spacing (in), rounded up, plus 1
Worked example (the calculator's default values)
A deck 16 ft long in the board direction and 12 ft wide, 5.5 in boards with a 1/8 in (0.125 in) gap, 16 ft boards, joists at 16 in on center, and 10 percent extra:
- Width in inches: 12 × 12 = 144 in
- Space per row: 5.5 + 0.125 = 5.625 in
- Rows: 144 ÷ 5.625 = 25.6, rounded up to 26
- Linear feet: 26 × 16 = 416 ft
- Boards: 416 × 1.10 = 457.6 ÷ 16 = 28.6, rounded up to 29
- Joists: 16 × 12 = 192 in ÷ 16 = 12, plus 1 = 13 joists
The calculator shows: "You need 29 deck boards (416 linear ft). 26 rows across the 12.0 ft width with a 0.125 in gap, plus 10% extra. Framing: about 13 joists at 16 in on center."
Row 26 is a partial row: 25.6 rows fill the width, so the last board gets ripped to about 60 percent of its width. If that leaves a skinny strip at the edge, nudge the gap within your maker's allowed range or rip the first and last boards evenly.
When the board length doesn't match the deck
The calculator divides total linear feet by board length, which assumes you can use the offcuts. That works when the deck length is a multiple of the board length, like a 16 ft deck with 16 ft boards or a 24 ft deck with 12 ft boards.
It doesn't always work otherwise. DCA 6 requires every piece of decking to bear on at least 3 joists, so a short offcut is often scrap. Take a 10 × 10 ft deck built with 12 ft boards: the calculator shows 21 boards, but each of the 22 rows needs its own board because the 2 ft offcuts are too short to use. In cases like this, count rows × boards per row (22 here) and add a few spares.
Rows by deck width
For 5.5 in wide boards, here's how the gap changes the row count:
| Deck width | Rows at 1/8 in gap | Rows at 3/16 in gap | Rows at 1/4 in gap |
|---|---|---|---|
| 8 ft | 18 | 17 | 17 |
| 10 ft | 22 | 22 | 21 |
| 12 ft | 26 | 26 | 26 |
| 14 ft | 30 | 30 | 30 |
| 16 ft | 35 | 34 | 34 |
| 20 ft | 43 | 43 | 42 |
Boards for common deck sizes
5.5 in boards, 1/8 in gap, 10 percent extra, joists at 16 in on center:
| Board run × deck width | Board length | Rows | Linear feet | Boards with 10% extra | Joists |
|---|---|---|---|---|---|
| 12 × 12 ft | 12 ft | 26 | 312 | 29 | 10 |
| 16 × 12 ft | 16 ft | 26 | 416 | 29 | 13 |
| 20 × 12 ft | 20 ft | 26 | 520 | 29 | 16 |
| 16 × 16 ft | 16 ft | 35 | 560 | 39 | 13 |
| 20 × 16 ft | 20 ft | 35 | 700 | 39 | 16 |
| 24 × 12 ft | 12 ft | 26 | 624 | 58 | 19 |
Joist spacing and gaps
| Item | Guidance | Source |
|---|---|---|
| Joist spacing, boards perpendicular | 16 in on center max for standard applications | Trex |
| Joist spacing, diagonal decking | 12 in on center | Trex |
| Stair treads (1 × 6 composite) | 12 in on center for Transcend, 9 in for Select and Enhance | Trex |
| Gap, side to side | About 1/8 in (wood); at least 3/16 in (Trex composite) | DCA 6; Trex |
| Gap, board ends | 1/8 in above 40°F, 3/16 in below 40°F | Trex |
| Gap against the house or posts | 1/4 in above 40°F, 1/2 in below 40°F | Trex |
| Support | Every piece of decking on at least 3 joists | DCA 6 |
Trex also notes that joists 12 in apart or closer may be needed for heavier loads, longer spans, or intricate patterns. Tighter spacing doesn't change the board count, only the joist count: the default deck needs 17 joists at 12 in on center instead of 13.
Fasteners
DCA 6 calls for 2 threaded 8d nails or 2 #8 screws at each joist. For face screws, multiply rows × joists × 2. The default deck works out to 26 × 13 × 2 = 676 screws, before a few spares. Hidden clip systems follow the decking maker's own count.
Tips and common mistakes
- Entering the nominal width. If you type 6 in for a board that's really 5.5 in, the default deck drops from 26 rows to 24, and you come up two rows short.
- Forgetting the extras. Picture-frame border boards, stair treads, and fascia aren't part of the field count. Figure them separately.
- Ignoring moisture in wood decking. Trex's spacing guide points out that moisture plays a big role with wood boards and suggests confirming a moisture content under 15 percent with a meter before installing.
- Putting composite on old wood framing. If existing joists are 24 in on center, Trex suggests adding a joist between each pair to get to 12 in spacing.
- Treating framing as an afterthought. Joist sizes, spans, beams, footings, and the ledger connection are structural. Follow DCA 6 or your local code, pull a permit where required, and hire a pro for the framing if you're unsure.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and a 1/8 in gap, a 12 ft wide deck takes 26 rows. Using 12 ft boards, that's 26 boards, or 29 with 10 percent extra, for 312 linear feet of decking.
How many deck boards do I need for a 16 × 16 deck?
35 rows of 16 ft boards (560 linear feet), or 39 boards with 10 percent extra. Framing at 16 in on center takes about 13 joists.
How much space should I leave between deck boards?
The American Wood Council's DCA 6 calls for decking spaced about 1/8 inch apart. Trex requires at least 3/16 inch side to side, plus 1/8 inch end to end above 40°F (3/16 inch below). Follow your decking maker's instructions.
How many joists do I need for a 12 × 16 deck?
If the boards run the 16 ft direction, joists are spaced along that 16 ft: 13 joists at 16 in on center, or 17 at 12 in on center for diagonal decking. Joist size and span come from DCA 6 or your local code.
How many screws do I need for deck boards?
DCA 6 calls for 2 screws (or 2 threaded nails) at every joist. Multiply rows × joists × 2. The 16 × 12 example is 26 × 13 × 2 = 676 screws before extras. Hidden fastener systems have their own counts.
Should I buy 12, 16, or 20 ft deck boards?
Pick a length that divides evenly into your deck length so rows need no short pieces. Trex sells 12, 16, and 20 ft boards. Where boards have to butt end to end, plan the joint over framing, and remember DCA 6 requires every piece to bear on at least 3 joists.