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.
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.
Rule of thumb for 6 inch nominal decking: plan on about 2.1 rows per foot of deck width. The calculator above 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 |
Deck boards by deck size: 10 x 10 to 20 x 20
29 boards for a 12 x 12 ft deck and 48 for a 20 x 20, with 5.5 in boards laid along the longer side and 10 percent extra. Each row gets one board, in the shortest of the 12, 16, and 20 ft lengths Trex and TimberTech sell that covers the run. The 1/8 in column uses DCA 6's spacing for wood; the 3/16 in column uses Trex's minimum gap for its composite boards:
| Deck size | Area | Board length | 1/8 in gap (wood) | 3/16 in gap (composite) |
|---|---|---|---|---|
| 10 x 10 ft | 100 sq ft | 12 ft (2 ft left over) | 22 rows, 25 boards | 22 rows, 25 boards |
| 10 x 12 ft | 120 sq ft | 12 ft | 22 rows, 25 boards | 22 rows, 25 boards |
| 12 x 12 ft | 144 sq ft | 12 ft | 26 rows, 29 boards | 26 rows, 29 boards |
| 12 x 16 ft | 192 sq ft | 16 ft | 26 rows, 29 boards | 26 rows, 29 boards |
| 12 x 20 ft | 240 sq ft | 20 ft | 26 rows, 29 boards | 26 rows, 29 boards |
| 14 x 14 ft | 196 sq ft | 16 ft (2 ft left over) | 30 rows, 33 boards | 30 rows, 33 boards |
| 16 x 16 ft | 256 sq ft | 16 ft | 35 rows, 39 boards | 34 rows, 38 boards |
| 16 x 20 ft | 320 sq ft | 20 ft | 35 rows, 39 boards | 34 rows, 38 boards |
| 18 x 18 ft | 324 sq ft | 20 ft (2 ft left over) | 39 rows, 43 boards | 38 rows, 42 boards |
| 20 x 20 ft | 400 sq ft | 20 ft | 43 rows, 48 boards | 43 rows, 48 boards |
On the 10 x 10, 14 x 14, and 18 x 18 decks each board is 2 ft longer than the run, and a 2 ft offcut can't reach the 3 joists 16 in apart that DCA 6 requires, so the count is one board per row plus 10 percent. The calculator assumes offcuts get used and shows fewer boards for those three sizes (21, 29, and 39); for the other sizes it gives the same count as the table. If your decking comes in a length that matches the run, enter that length instead.
Square feet per deck board
7.5 sq ft for a 16 ft board of 5.5 in decking with a 1/8 in gap: each board covers its length times its width plus one gap (16 × 5.625 ÷ 12). That's 2.13 linear ft of decking per sq ft of deck, or 2.11 at a 3/16 in gap, so 100 sq ft takes about 213 linear ft before extra. Trex says board lengths usually range from 8 to 20 ft:
| Board length | Sq ft per board, 1/8 in gap | Sq ft per board, 3/16 in gap | Boards per 100 sq ft, 1/8 in gap | Boards per 100 sq ft, 3/16 in gap |
|---|---|---|---|---|
| 8 ft | 3.75 | 3.79 | 26.67 | 26.37 |
| 10 ft | 4.69 | 4.74 | 21.33 | 21.1 |
| 12 ft | 5.63 | 5.69 | 17.78 | 17.58 |
| 16 ft | 7.5 | 7.58 | 13.33 | 13.19 |
| 20 ft | 9.38 | 9.48 | 10.67 | 10.55 |
Home Depot's square-footage method divides the deck area by the area of one board, then adds 10 percent. It can come up a board short, because the last row takes a whole board even when you rip it: the 16 × 12 ft default deck works out to 192 ÷ 7.5 = 25.6 boards, but it has 26 rows. Use the chart as a check and order from the row count the calculator gives.
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.
Gap and joist spacing by decking type
16 in on center for 5/4 wood and 1 in composite boards laid square to the joists, and 12 in when the boards run at 45 degrees. The wood rows come from the residential code's decking table (Table R507.7) and DCA 6, the composite rows from the Trex and TimberTech installation guides:
| Decking | Gap between boards | Joists, boards square to joists | Joists, boards at 45° |
|---|---|---|---|
| Wood, 5/4 (1 1/4 in thick) | About 1/8 in | 16 in on center | 12 in on center |
| Wood, 2 in thick (2 x 6) | About 1/8 in | 24 in on center | 16 in on center |
| Trex 1 in and Select boards | At least 3/16 in | 16 in on center | 12 in on center |
| Trex 2 x 6 boards | At least 3/16 in | 24 in on center | 20 in on center |
| TimberTech composite | 1/8 to 3/16 in | 16 in on center | 12 in on center |
Trex shortens the span by 2 in at 60 degrees and halves it at 30 degrees, so its 1 in boards need joists 8 in apart at a 30 degree angle. The code limits wood decking to 45 degrees from perpendicular, and composite boards follow the maximum span on their label. A picture-frame border needs extra framing too: Trex adds two joists on each side of the deck, one under the inside edge of the border and one under the ends of the field boards, so order four or more. On the 16 ft default deck, the calculator counts 9 joists at 24 in on center, 13 at 16 in, and 17 at 12 in.
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.
Screws and hidden clips per square foot
About 3.5 face screws or 1.8 hidden clips per sq ft, with joists 16 in on center. Screws go 2 per board at each joist (DCA 6, Trex). Clip boxes list the area they cover, so Home Depot's method is to divide the deck area by the coverage on the box and round up:
| Fastener | Count | Per sq ft | 16 x 12 ft deck (192 sq ft) |
|---|---|---|---|
| Face screws, joists 16 in on center | 2 per board at each joist | 3.52 | 676 |
| Face screws, joists 12 in on center | 2 per board at each joist | 4.6 | 884 |
| Trex Hideaway clips | 90 per box, covering 50 sq ft at 16 in on center | 1.8 | 4 boxes (360 clips) |
| TimberTech CONCEALoc clips | 175 per box, covering 100 sq ft | 1.75 | 2 boxes (350 clips) |
Clips hold each board at each joist, so the default deck has 26 rows × 13 joists = 338 crossings, 1.76 per sq ft, in line with the boxes. With joists 12 in apart, a box rated at 16 in on center covers about 37.5 sq ft (50 × 12 ÷ 16), so the same deck takes 6 boxes of Hideaway. Add the area of the stair treads, as Home Depot notes, and the starter clips the maker lists for the first row.
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 12x12 deck?
29 boards of 12 ft with 10 percent extra, or 26 without. With 5.5 in boards and a 1/8 in gap, a 12 ft wide deck takes 26 rows, for 312 linear feet of decking. Composite boards at Trex's 3/16 in gap also fit 26 rows across 12 ft.
How many deck boards do I need for a 12x16 deck?
29 boards of 16 ft with 10 percent extra, if the boards run the 16 ft side: 26 rows of 5.5 in boards with a 1/8 in gap, or 416 linear feet. That's the calculator's default deck. Run the boards the 12 ft way and it takes 35 rows of 12 ft boards (420 linear feet), or 39 with 10 percent extra.
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 many composite deck boards do I need?
For a 16 × 12 ft deck, 29 boards of 16 ft with 10 percent extra, the same as wood. Trex requires a gap of at least 3/16 in side to side, but with its 5.5 in wide boards that still makes 26 rows across 12 ft (416 linear feet). On some widths the wider gap drops a row: a 16 × 16 ft deck takes 34 rows and 38 boards at 3/16 in, against 35 rows and 39 boards at 1/8 in. Enter 0.1875 as the gap.
How many 5/4 deck boards do I need?
The same as 2 × 6 boards: 29 boards of 16 ft for a 16 × 12 ft deck, with a 1/8 in gap and 10 percent extra. The 5/4 is the nominal thickness; the Southern Forest Products Association lists 5/4 × 6 radius edge decking at 1 × 5 1/2 in dressed, so enter 5.5 in as the board width. DCA 6 allows decking thinner than 2 in nominal only if it is span-rated, so check the product's span before you set the joist spacing.
How much space should I leave between deck boards?
About 1/8 in for wood and 1/8 to 3/16 in for composite, depending on the brand. 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), while TimberTech's composite guide calls for 1/8 to 3/16 inch side to side. 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.
How many deck screws do I need per square foot?
About 3.5 per sq ft, or roughly 350 per 100 sq ft, with 5.5 in boards and joists 16 in on center. DCA 6 and Trex's installation guide both call for 2 screws per board at each joist, so the 16 × 12 ft default deck takes 26 rows × 13 joists × 2 = 676 screws for 192 sq ft, or 3.52 per sq ft. With joists 12 in apart, it is 884 screws, about 4.6 per sq ft. Hidden clips follow the clip maker's coverage.
How many hidden deck fasteners do I need per square foot?
About 1.8 per sq ft with joists 16 in on center: a box of 90 Trex Hideaway clips covers 50 sq ft, and TimberTech packs 175 CONCEALoc clips for 100 sq ft (1.75 per sq ft). That's about one clip per board at each joist, so the 16 × 12 ft default deck (26 rows × 13 joists = 338) takes 4 boxes of Hideaway or 2 of CONCEALoc. With joists 12 in apart, count a 50 sq ft box as about 37.5 sq ft.
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.
How much extra decking should I buy?
10 to 15 percent for boards laid straight, and 30 percent for diagonal or herringbone patterns, according to TimberTech. The calculator's default is 10 percent, the same as Home Depot's guide, and Trex's rule of thumb is 5 percent. For the 16 × 12 ft default deck, that's 28 boards of 16 ft at 5 percent, 29 at 10, 30 at 15, and 34 at 30 percent.
What is the actual width of a 2x6 deck board?
5 1/2 inches. The Southern Pine Inspection Bureau notes that 2 x 4s and 2 x 6s are 1/2 inch narrower than their nominal width, and Trex's 1 x 6 profile boards are also 5.5 in wide. Entering 6 in instead of 5.5 in drops the default deck from 26 rows to 24, which leaves you two rows short. Board feet, by contrast, are figured on the nominal 2 x 6 size; see the board foot calculator.
How many square feet does a deck board cover?
7.5 sq ft for a 16 ft board, 5.63 sq ft for a 12 ft board, and 9.38 sq ft for a 20 ft board, with 5.5 in decking and a 1/8 in gap. Each board covers its length times its width plus one gap: 16 × 5.625 ÷ 12 = 7.5. Dividing the deck area by that figure can leave you a board short, because the last row takes a whole board even when it's ripped: the 16 × 12 ft deck works out to 25.6 boards but has 26 rows.
How far apart should deck joists be?
16 in on center for most decking laid square to the joists, and 12 in for diagonal decking. Trex lists 16 in on center as the maximum for standard applications with boards perpendicular to the joists, and 12 in on center for diagonal decking; TimberTech's composite guide sets the same limits. The residential code's Table R507.7 allows 24 in (16 in diagonal) for 2 in thick wood. For composite over existing joists at 24 in on center, Trex suggests adding a joist between each pair. Joist sizes and spans come from DCA 6 or your local code.
How many deck boards do I need for a 10 x 10 deck?
With 5.5 in boards and a 1/8 in gap, a 10 ft wide deck takes 22 rows. If you use 12 ft boards, the 2 ft offcuts are too short to reuse, because DCA 6 requires every piece to bear on at least 3 joists, so plan on 22 boards plus a few spares.
Sources
- American Wood Council: DCA 6, Prescriptive Residential Wood Deck Construction Guide (PDF)
- Trex: 2023 Decking Installation Guide (PDF)
- Trex: How to Easily Space Deck Boards
- Trex: How to Space and Install Deck Joists
- Southern Pine Inspection Bureau: Nominal vs. Actual Lumber Sizes
- Southern Forest Products Association: Standard Sizing
- TimberTech: Composite Decking Installation Guide (PDF)
- Trex: How Many Deck Boards Do I Need?
- The Home Depot: How to Calculate Deck Materials
- City of New Prague, Minnesota: Section R507 Exterior Decks, 2020 Minnesota Residential Code (PDF)
- Trex: How to Picture Frame a Deck Like a Pro
- Trex: Hideaway Universal Hidden Fasteners
- TimberTech: CONCEALoc Hidden Fastener
- TimberTech: How to Calculate How Much Decking You Need
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