Decking

How Many Deck Boards Do You Need (And Why Brand Actually Matters)

I built the deck board calculator after sourcing Trex and TimberTech numbers for the tool and watching the “obvious” gap and width figures disagree with each other — and, in Trex’s case, with Trex’s own pages. Brand isn’t a marketing toggle here; it changes the row math.

The math, worked through

Board count comes down to three numbers: your deck’s width, the board’s actual face width, and the gap between boards.

Rows across the deck = deck width ÷ (board width + gap), rounded up. Multiply that by the deck’s length to get total linear footage, add your waste percentage, then divide by the board length you’re buying.

The board width itself is where people get tripped up. A “1×6” or “5/4×6” board isn’t actually 6 inches wide — nominal lumber sizing has never matched actual dimensions. The real, dressed width is 5.5 inches, per the American Softwood Lumber Standard. Composite boards run the same 5.5-inch width in most product lines, though it’s always worth checking the specific line you’re buying rather than assuming. We reversed an early TimberTech draft that used 5.36 inches from a 2019 PDF whose URL now 404s; current live TimberTech pages and retail listings say 5.5 inches, so that’s what the calculator ships.

Square footage alone undercounts, because it ignores the gap. A 5½-inch board plus a 1/8-inch gap occupies 5.625 inches of width — fewer rows than a bare area figure suggests.

Where the online calculators quietly disagree

I hit this while building the presets: same nominal deck, different tools, different board counts — and when I chased the gap figures back to manufacturer pages, Trex itself didn’t agree with Trex.

Here’s something worth knowing if you’re comparing this tool’s numbers to someone else’s: gap spacing isn’t as settled as it looks.

Standard lumber convention is 1/8 to 1/4 inch, and it genuinely depends on whether your boards are wet pressure-treated (which shrinks as it dries, so installers run it tighter) or kiln-dried (which doesn’t move much, so 1/8 inch is standard).

Trex is a more interesting case. Their own installation guide for hidden fasteners specifies 3/16 inch. But a separate page on their own site — a step-by-step joist installation guide, with the math worked out in front of you — states the gap as 1/4 inch and uses that figure to calculate board count (5.5 + 0.25 = 5.75 inches per row). Two pages, same company, two different numbers.

We didn’t pick one arbitrarily. We used the 1/4 inch figure because it’s the one we could verify against a live page with the actual math shown, not because it’s definitively “more correct” than the other. The discrepancy is called out on the calculator page and in the FAQ. If you’re using a specific Trex fastening system, check your product’s current installation guide — manufacturers update these, and even they aren’t always internally consistent.

The number that actually moves your budget

Board gap differences are usually small — a few boards, maybe, across a typical deck. Joist spacing is bigger.

Standard spacing for most composite and wood decking is 16 inches on center. If you’re laying boards diagonally, that tightens to 12 inches on center — a 25% reduction in spacing, which works out to roughly a third more joists across the same frame. That’s real lumber cost, not a rounding difference, and it’s the kind of thing worth knowing before you fall in love with a herringbone pattern. Some products go further: TimberTech Advanced PVC wants 12 inches even for straight runs, so the product sheet still wins over a generic “composite = 16 inches” rule.

What actually changes your answer

• Wet vs. dry lumber. Pressure-treated wood installed wet will gap wider as it dries — installers often run boards nearly tight for this reason, understanding the gap will open up over the following months.
• Layout pattern. Diagonal or picture-frame borders both add material waste beyond a straight run — budget 15% or more rather than the standard 10%.
• Brand-specific hardware. Hidden fastener systems (grooved-edge boards) lock in a specific gap by design — check the fastener spec, not just the board spec, if you’re using one.
• Board length vs. run length. A 17-foot run on 16-foot boards means a butt joint and a short filler on every row, and every joint has to land on a joist. Buy the longest board that spans the deck in one piece when you can.

One practical tip for the lumber yard: price the board count at both gap presets you’ll actually consider — 1/8 inch for dry lumber, 1/4 inch for Trex’s live planning math — before you commit to a composite vs. wood decision. The brand choice isn’t just the sticker price per linear foot; it’s which gap the manufacturer’s current guidance puts into the row divisor.

Width, gap, and joist-spacing presets on the calculator are cited on that page’s sources block. The fuller sourcing rules — live URLs only, reverse a number when the PDF dies — are on the methodology page. If a current product sheet disagrees with what we ship, say so on the contact page.