How the boards are counted
The boards run along the deck’s length in rows laid side by side across its width. The calculator works out how many rows cover the width with the maker’s gap between them, how many boards each row takes with a gap at every butt joint, and then adds the extra you choose. Everything is in inches.
Worked example: a 16 × 12 ft deck of 16 ft Trex boards
- Width to cover: 12 ft is 144 in. A board and its gap: 5½ + 3/16 = 5 11/16 in (5.6875).
- Rows: (144 + 0.1875) ÷ 5.6875 = 25.35, so 26 rows.
- Last board: 144 − 25 × 5.6875 = 1.8125, so the last row is ripped to 1 13/16 in.
- Per row: one 16 ft board covers the 16 ft run, so there are no butt joints and 26 boards in all.
- Extra: 26 × 5 ÷ 100 = 1.3, rounded up to 2. Buy 28 boards, for 26 × 16 = 416 linear ft of deck.
Trex’s own example leaves the gaps out: 144 ÷ 5.5 = 26.18, so 27 rows, and 5% of 27 is 1.35, rounded up to 2, for 29 boards.
Board length and butt joints
When the deck is longer than the boards, each row takes two or more boards end to end, and every place two ends meet is a butt joint. Both ends need support, so each joint goes over a doubled joist or blocking. Trex says every board must span at least three joists, so a short offcut can’t finish a row. Stagger the joints so they don’t line up from row to row.
A 24 ft deck of 16 ft Trex boards at 70 °F takes (288 + 1/8) ÷ (192 + 1/8) = 1.4997, so 2 boards a row. Across 26 rows that is 52 boards and 26 butt joints, and 55 boards with 5% extra. Using 12 ft boards also takes 2 a row, since two of them and a 1/8 in gap make 24 ft 1/8 in: still 52 boards, but 624 linear ft of decking instead of 832, with only the ends to trim.
Trex counts the other way: rows × deck length ÷ board length, as if every offcut were used. For a 12 × 12 ft deck, 27 rows × 12 ft = 324 linear ft, which is 27 boards of 12 ft, 21 of 16 ft (20.25) or 17 of 20 ft (16.2). The 16 ft and 20 ft counts only work if the offcuts join up into rows, which puts a butt joint in every row. The calculator shows both: the board-by-board count as the main number, and the pooled count under “If offcuts are reused,” with the same extra added.
Gaps by brand and temperature
Composite boards grow and shrink in length with the temperature, so the makers set the end gap by how warm it is when you lay them; the side gap stays the same. The calculator uses the end gap for the temperature you enter.
| Boards | Side gap | End gap at butt joints |
|---|---|---|
| Trex | 3/16 in; 3/8 in for wooded areas | 1/8 in above 40 °F; 3/16 in at 40 °F or colder |
| TimberTech composite | 1/8 to 3/16 in (CONCEALoc clips set 3/16 in) | 3/16 in at 32 °F or colder; 1/8 in from 33 to 74 °F; 1/32 in at 75 °F or warmer |
| TimberTech Advanced PVC | 1/8 to 1/4 in | None |
| Fiberon Concordia | At least 3/16 in | 1/4 in at 35 °F or colder; 3/16 in from 36 to 69 °F; 1/8 in from 70 to 89 °F; 1/16 in from 90 to 109 °F; none at 110 °F or warmer |
| Wood | About 1/8 in (AWC DCA 6) | — |
Where boards meet the house or a post, Trex leaves 1/4 in above 40 °F and 1/2 in at 40 °F or colder, and Fiberon sets its starter clips at least 1/4 in from the wall. Fiberon’s end gaps are from its 1/22 instructions; the 2/20 edition gave 1/32 in at 110 °F or warmer instead of none. TimberTech notes that freshly treated wood goes in tight, because it shrinks as it dries. The deck board spacing guide goes through the gaps in more detail.
How much extra to allow
Trex adds 5% to its board count. TimberTech suggests 10 to 15% for boards laid straight and 30% for diagonal or herringbone layouts. The extra is rounded up to whole boards.
TimberTech’s example is a 16 × 12 ft deck of 16 ft boards 0.45 ft (5.4 in) wide, with no gap counted: 27 boards. Adding 10% is 2.7, rounded up to 3, for 30 boards; 15% is 4.05, rounded up to 5, for 32. To check it here, choose “Other boards,” enter 5.4 in with a 0 gap and set the extra to 10%.
The calculator counts boards laid square to the joists, so for a diagonal deck use TimberTech’s 30% as the extra. Diagonal boards also need closer joists: TimberTech asks for 12 in on center for any angled install, and Fiberon for 12 in under diagonal boards.
Board sizes, lengths and joist spacing
The width that matters is the actual width, not the nominal size: a wood 2×6 is 1½ × 5½ in. The calculator uses these sizes and lengths:
| Boards | Actual size | Lengths |
|---|---|---|
| Trex 1×6 | 0.94 × 5.5 in | 12, 16, 20 ft |
| TimberTech composite | 5.36 in wide (Premier 0.89 in thick) | 12, 16, 20 ft |
| Fiberon Concordia | 0.94 × 5.4 in | 12, 16, 20 ft |
| Wood 5/4×6 | 1 × 5.5 in | Whole feet; 8 to 20 ft here |
| Wood 2×6 | 1.5 × 5.5 in | Whole feet; 8 to 20 ft here |
Wood sizes are Southern Pine’s; softwood lumber is sold in whole-foot lengths under PS 20, and the calculator offers even lengths from 8 to 20 ft. Brand names identify products only.
Trex wants joists 16 in on center under its 1× boards on a home’s deck and 12 in on commercial decks. TimberTech allows 16 in at most, and 12 in for any angled or commercial install. Fiberon calls for 16 in on homes, 12 in for diagonal boards and 12 in on stairs. The deck fastener calculator counts the clips or screws for the same deck from its joist spacing.
Questions people ask
How many deck boards do I need for a 12×12 deck?
With Trex’s 5½ in boards and 3/16 in gaps, (144 + 3/16) ÷ 5 11/16 = 25.35, so 26 rows. With 12 ft boards that is 26 boards, and 28 with Trex’s 5% extra (1.3, rounded up to 2). Trex’s own example leaves the gaps out and gets 27 rows: 27 boards of 12 ft, or 21 of 16 ft if the offcuts are reused.
How many deck boards for a 16×12 deck?
With the boards running the 16 ft way: 26 rows of 16 ft Trex boards, and 28 to buy with 5% extra. The last row is ripped to 1 13/16 in. TimberTech’s example for the same deck, with 5.4 in boards and no gap, comes to 27 boards, then 30 with 10% extra or 32 with 15%.
What gap should I leave between composite deck boards?
Trex wants 3/16 in between boards at any temperature, and 3/8 in for decks in wooded areas. TimberTech allows 1/8 to 3/16 in, and its CONCEALoc clips set 3/16 in. Fiberon asks for at least 3/16 in. For wood, the American Wood Council’s DCA 6 spaces decking boards about 1/8 in apart.
Do the ends of composite deck boards need a gap?
Yes, where two ends meet at a butt joint, and the gap depends on the temperature as you lay them. Trex: 1/8 in above 40 °F, 3/16 in at 40 °F or colder. TimberTech: 3/16 in at 32 °F or colder, 1/8 in from 33 to 74 °F, 1/32 in at 75 °F or warmer. Fiberon: from 1/4 in at 35 °F or colder down to none at 110 °F or warmer. TimberTech’s Advanced PVC boards need no end gap.
How much extra decking should I buy?
Trex adds 5%. TimberTech suggests 10 to 15% for boards laid straight and 30% for diagonal or herringbone layouts. On a 26-board deck those come to 2 extra boards at 5%, 3 at 10% (2.6, rounded up) and 8 at 30% (7.8, rounded up).
Can I mix board lengths on one deck?
Yes. A 28 ft row can be a 12 ft and a 16 ft board, trimmed by the 1/8 in end gap. Swap their order from row to row so the joints don’t line up, put every joint over a doubled joist or blocking, and keep each piece long enough to span at least three joists, as Trex requires. The calculator counts one length at a time.
Sources for the numbers on this page
- Trex — How many deck boards do I need? · checked October 3, 2026
- Trex — Decking installation guide, 2026 edition (gaps, spans, fastening, stairs) · checked October 3, 2026
- Trex — How to space deck boards · checked October 3, 2026
- TimberTech — How to calculate how much decking I need · checked October 3, 2026
- TimberTech — Composite decking installation guide, REV 09/23 · checked October 3, 2026
- TimberTech — Deck board spacing · checked October 3, 2026
- TimberTech — 2023 product guide (board sizes and lengths, PDF) · checked October 3, 2026
- TimberTech — Advanced PVC decking installation with TOPLoc, REV 09/23 (PDF) · checked October 3, 2026
- Fiberon — Concordia decking (board dimensions) · checked October 3, 2026
- Fiberon — PE decking fastening instructions, FIB-0354-LIT 1/22 (distributor copy, PDF) · checked October 3, 2026
- American Wood Council — DCA 6: Prescriptive residential wood deck construction guide (2015 IRC edition) · checked October 3, 2026
- Southern Forest Products Association — Southern Pine standard sizes (nominal and actual dimensions) · checked October 3, 2026
- National Institute of Standards and Technology — PS 20-10, American Softwood Lumber Standard (standard lengths, PDF) · checked October 3, 2026
Corrections and updates are logged on the sources page.