A 12 ft by 12 ft deck with 5.5 inch wide boards needs 29 boards by the Trex method: 144 inches / 5.5 = 26.18, round up to 27, add 5% for waste (2 boards) and you order 29.1 If you account for the 3/16 inch gap Trex requires between boards,2 the same deck needs 26 rows and 28 boards ordered. These are row-count estimates before end trimming, perimeter gaps, borders or a cutting plan. This guide shows both methods and a chart for common sizes.
The formula
- Decking boards run in rows across the deck. Number of rows = deck depth (in) / board width (in), rounded up. Trex uses 5.5 inches for its boards.1
- Boards to buy = rows + 5%. Trex says the general rule is 5% extra for waste and mistakes.1
- Our chart budgets one nominal stock board per row when stock length matches the run, before trimming and end-gap checks. Trex boards come in 12, 16 and 20 ft lengths.1
- With the gap: rows = round up ((deck depth in inches + gap) / (board width + gap)), because N rows have N – 1 gaps. Trex requires a 3/16 inch width-to-width gap at all temperatures.2 So 5.5 + 0.1875 = 5.6875 inches per row.
Trex’s own worked method divides by 5.5 inches only. Using the board plus the gap gives a slightly lower count on larger decks, so we show both. The gap figure is for Trex boards. Check the installation guide for the boards you buy.
The two installation documents linked below disagree on minimum side spacing: the 2026 guide shows 3/16 inch in its layout diagrams, while the older chapter-four document says 1/4 inch in its width-to-width text. This chart uses the 2026 diagram value as an estimating example, not permission to override the current product and fastening instructions. Check the guide for your exact boards and fastener system before setting the gap.2,3
Chart: boards to order by deck size (5.5 inch boards, 5% extra)
| Deck (length x depth) | Board length | Trex method (no gap) | With 3/16 in gap |
|---|---|---|---|
| 12 x 10 ft | 12 ft | 22 rows, order 24 | 22 rows, order 24 |
| 12 x 12 ft | 12 ft | 27 rows, order 29 | 26 rows, order 28 |
| 16 x 14 ft | 16 ft | 31 rows, order 33 | 30 rows, order 32 |
| 20 x 16 ft | 20 ft | 35 rows, order 37 | 34 rows, order 36 |
Depth is the distance across the boards. Counts budget one nominal stock board per row, before trimming and end-gap checks, and extra boards are rounded up to a whole board. Stairs, fascia and picture frame borders are extra.
Worked example 1: Trex’s 20 x 12 ft arithmetic example
- Depth: 12 ft x 12 = 144 inches.
- 144 / 5.5 = 26.18, round up to 27.
- 5% of 27 = 1.35, round up to 2 extra.
- Order 29 boards. Trex applies this arithmetic to a 20 x 12 ft deck; only the 12 ft row span enters this row count. Its article prints the stock length as 20 inches, a unit typo inconsistent with its deck dimensions and listed lengths. This matches the published row count, not a verified cutting list.1
Worked example 2: the same deck, with the 3/16 inch gap
- Row width: 5.5 + 0.1875 = 5.6875 inches.
- (144 + 0.1875) / 5.6875 = 25.3516, round up to 26.
- 5% of 26 = 1.3, round up to 2 extra.
- Order 28 boards.
Worked example 3: a 16 x 14 ft deck, 16 ft boards
- Depth: 14 x 12 = 168 inches.
- Trex method: 168 / 5.5 = 30.55, round up to 31 rows, plus 2 extra = 33.
- With the gap: (168 + 0.1875) / 5.6875 = 29.5714, round up to 30, plus 2 extra = 32.
- Square cut ends need an end-to-end gap too. Trex’s installation guide gives 1/8 inch above 40 degrees F and 3/16 inch below it.3
Fasteners and framing
This count covers surface boards. TimberTech’s guide also goes through board square footage and waste,4 but joists, fasteners and railings are separate. Our deck board calculator counts boards for a given size.
Common mistakes
- Dividing area by area per board and ignoring the spacing between boards.
- Forgetting the waste allowance for mitered cuts and angled decks.1
- Buying a board length that makes a lot of cutoff. A 14 ft deck on 16 ft boards wastes 2 ft per row.
- Treating the 3/16 inch gap as universal. It is Trex’s figure.2
FAQ
How many deck boards do I need for a 12×12 deck?
29 by the Trex method, or 28 if you count the 3/16 inch gap.
How much extra decking should I buy?
Trex gives 5% as a general rule.1
What length boards come in?
Trex boards come in 12, 16 and 20 ft lengths.1
Match the guide and calculator settings
Our deck board calculator uses the gap-aware formula and starts with a 10 percent example allowance and a 1/8-inch example gap. To reproduce this guide’s gap-aware chart, enter 0.1875 for the side gap and 5 for the extra percentage. Different allowances create different orders: 26 rows with 5 percent extra rounds to 28 boards; with 10 percent it rounds to 29. Neither percentage replaces a detailed cutting list.
The last row does not have another side gap
N rows cover N x board width + (N – 1) x gap. Solve that expression for N to get (target width + gap) / (board width + gap), then round up. The old width / (board + gap) shortcut gives the same whole-row counts for the chart above, but can undercount at a boundary. For an example target span of 113.75 inches, 20 boards of 5.5 inches with 19 gaps of 0.1875 inches cover 113.5625 inches. The shortcut would say 20 rows, but that is short of the target; the corrected formula calls for 21 rows. Perimeter clearances still need separate planning.
Stock length is not always finished usable length
A deck run that matches a nominal board length still needs a trimming and gap check. The 2026 Trex guide linked here requires clean cutting at least 3/16 inch from both ends. Do not assume a stock board exactly 16 feet long becomes a usable 16-foot finished piece. Actual delivered length, end trims, required abutting gaps and overhang determine the final cut. An extra waste percentage does not make an individual short board span a longer run.2
For rows longer than your usable stock, decide where butt joints land and whether offcuts can be reused before ordering. Our calculator conservatively rounds stock pieces up for every row; it does not optimize reuse or lay out the supporting joists. Diagonal patterns, borders, stairs and mixed board widths need their own takeoff. Trex’s 5 percent suggestion is a general rule, not a guarantee for those layouts.1
Related tools and guides
Deck board calculator, decks hub, all calculators, fence post spacing.
Sources
- Trex, How Many Deck Boards Do I Need? (5.5 inch boards, 12/16/20 ft lengths, 5% waste, 20 x 12 deck example; source has a board-length unit typo)
- Trex Decking Installation Guide (2026) (3/16 inch width-to-width gap)
- Trex, Decking Installation Guide, chapter four (end-to-end gap 1/8 inch above 40F, 3/16 inch below)
- TimberTech, How to Calculate How Much Decking You Need (board square footage, spacing and waste steps)