Why does the gap between deck boards affect how many boards I need?

Because a board's real coverage is its width plus the gap beside it, not its width alone. A 12 ft, 5.5 in. board at a 3/16 in. gap covers 5.69 sq ft; the same board at a 3/8 in. gap covers 5.875 sq ft — about 3.3% more. Across a whole deck, that difference moves your board count by roughly the same percentage.

What gap should I use between deck boards?

Whatever your manufacturer's installation instructions specify — this page won't guess, because real manufacturers genuinely disagree. Trex recommends 3/16 in. to 3/8 in. for its composite boards. TimberTech's guidance is typically 1/8 in. to 1/4 in. Both are correct for their own products; neither is a universal number.

Why do wood and composite decking need different spacing?

Because they expand and contract in different directions. Traditional wood moves primarily across its width, so the width-to-width gap does most of the work. Composite moves primarily along its length, which is why the gap at butt joints (where boards end) matters more for composite than it does for wood.

Should pressure-treated decking be installed with a gap?

It depends on whether the lumber is wet or dry when it's installed, a distinction worth checking with your supplier rather than assuming. Wet PT lumber is often installed tight, or nearly so, because it shrinks as it dries and creates its own gap over time. Dry PT lumber doesn't get that free shrinkage and needs a real gap left on purpose.

What board length should I use?

There's no single right answer — it's a tradeoff. Longer boards (16 or 20 ft, alongside the common 12 ft length) mean fewer butt joints and less offcut per foot, but a length that doesn't divide evenly into your deck's dimension can leave an unusable drop. Check your options against your deck's actual dimensions rather than defaulting to the longest board available.

Does this calculator cover joist spacing, footings, or railing?

No, deliberately. Joist spacing, beam sizing, post spacing, footings, ledger attachment, and railing are all governed by the IRC's prescriptive deck-construction provisions, where a wrong number is a structural safety risk, not just an over-order. This calculator counts deck boards only — get those six from the current IRC and your local building department.

Why does this calculator show results in linear feet as well as a board count?

Because some suppliers quote and sell decking by the linear foot rather than by the piece. Linear feet here is simply the board count multiplied by your chosen board length — the same order, in the units your supplier might actually ask for.

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Deck Board Calculator

A deck board doesn't cover its own nominal width — it covers its width plus the gap beside it, and skipping that second number is the most common way a deck board order comes up short. This calculator asserts almost nothing of its own: the coverage-per-board formula is pure geometry, board length offers common stock sizes but stays open to any custom length, and both your board width and your gap are numbers you bring with you, since gap especially varies by product and this page won't guess between manufacturers that genuinely disagree. Enter your deck's area, pick or type a board length, confirm your board's actual width, and enter the gap your product's instructions call for — the results show your deck's area, the amount to order after waste, a board count, and the same order in linear feet. This page also draws a hard line around what it doesn't cover: joist spacing, beam sizing, post spacing, footings, ledger attachment, and railing are all governed by the IRC's structural provisions, not estimated here — see the guidance below for why, and where to actually get those numbers.

Your deck

LengthWidthUnit

Board length (ft)

Board width (in)Gap (in)

Trex recommends 3/16 in. to 3/8 in. between composite boards, wider for more debris clearance in wooded settings. TimberTech's guidance is different — typically 1/8 in. to 1/4 in., because different materials expand and contract in different directions. Neither range is wrong; they're different products. Use the spacing your manufacturer's own instructions specify, not a number averaged between two companies that disagree on purpose.

Waste (%)10% is a starting point, not a sourced figure — this site hasn't found a published decking-specific waste percentage from a manufacturer or trade authority worth citing. What actually drives waste is your layout: a picture-frame border, a diagonal pattern, or a deck whose dimension isn't a clean multiple of your board length all consume more than a straight run does (see the guidance below on board length and waste). A contractor bidding your job, or your decking product's own installation guide, is where a real number for your specific layout lives.

Enter your deck dimensions above to see results.

How it's calculated

The calculator first resolves your deck area from either dimensions (length × width) or a direct entry, then computes each board's real coverage as board length (ft) × (board width + gap, in inches) ÷ 12 — the step that accounts for the gap a nominal-width estimate misses. A waste percentage is added to the deck area (10% by default — see the guidance below for why that's a starting point, not a sourced figure), and the waste-adjusted area is divided by the per-board coverage to get a board count, rounded up since a partial board still has to be bought. Linear feet is that board count multiplied by your chosen board length — the same order, expressed the way a supplier who sells by the foot would quote it.

Worked example

A 16 ft × 12 ft deck (192 sq ft), 16 ft boards, 5.5 in. wide, at a 3/16 in. gap and the 10% waste default: coverage per board = 16 × (5.5 + 0.1875) ÷ 12 = 16 × 5.6875 ÷ 12 = 7.58 sq ft. Waste-adjusted area = 192 × 1.10 = 211.2 sq ft. Boards needed = 211.2 ÷ 7.58 = 27.85 — rounded up to 28 boards. In linear feet, that's 28 × 16 = 448 ft. Change any of the four inputs — deck size, board length, board width, or gap — to your actual project's numbers; every one of them is yours to set, not this page's.

Gap, board length, waste, and what's left out

A deck board's nominal width isn't what it actually covers once it's installed — it covers its own width plus the gap left beside it for drainage, airflow, and material movement. A 12 ft board of 5.5 in. decking at a 3/16 in. gap occupies 5.6875 in. of deck width, so it covers 5.69 sq ft, not the 5.5 sq ft its nominal width suggests. At a 3/8 in. gap, the same board covers 5.875 sq ft — about 3.3% more. That difference compounds across an entire deck: between Trex's own minimum (3/16 in.) and maximum (3/8 in.) recommended gap, the board count for the same deck moves by roughly that same 3.3%. Most rough estimates ignore gap entirely and quietly under-order as a result.

Wood and composite decking need genuinely different spacing, and the reason is a real material-science difference, not a preference. Traditional wood expands and contracts primarily across its width as moisture changes — which is why wood decking is typically installed with a consistent width-to-width gap and needs comparatively little allowance at the board ends (butt joints). Composite decking behaves close to the opposite: it expands and contracts primarily along its length. That's why butt-joint gaps matter a great deal for composite — leaving room at the ends where boards meet — and comparatively little for wood, where the width gap is doing most of the work. Mixing up which gap matters for which material is an easy, avoidable mistake.

Pressure-treated lumber adds one more wrinkle this page states as a general distinction to check with your supplier, not as a number — no citation backs the specifics, only the general principle. Lumber that's still wet when it arrives on a truck is often installed with the boards touching, or nearly so, because it shrinks as it dries and creates its own gap over the following weeks. Lumber that's already dry when installed doesn't get that free shrinkage, and needs a real gap left on purpose from day one. Installing either one as if it were the other — a tight gap on dry lumber, or a full gap on wet lumber that then shrinks further than expected — is a common, avoidable mistake. Ask your lumber supplier whether what you're buying is wet or kiln-dried before deciding your gap.

Gap recommendations genuinely differ by manufacturer, and that's worth taking at face value rather than averaging away. Trex recommends 3/16 in. to 3/8 in. between composite boards — wider gaps let more organic debris pass through in wooded settings. TimberTech's guidance is narrower, typically 1/8 in. to 1/4 in., and TimberTech is explicit that recommendations aren't universal because different materials expand and contract in different directions. Neither company is wrong; they're describing different products. This calculator's gap field has no default for exactly this reason — the number that's correct for your deck comes from your product's own installation instructions, not a figure this page could assert on your behalf.

Board length affects waste in a way that's easy to miss. Longer boards mean fewer butt joints and less offcut per linear foot of decking, which pushes actual waste down. But a board length that doesn't divide evenly into your deck's dimension in the direction the boards run creates its own offcut — a 20 ft board on a run that needs 13 ft leaves a 7 ft drop that may or may not be usable elsewhere on the job. There's no single right board length; it's a tradeoff between fewer joints and more potential drop, worth thinking through against your deck's actual dimensions rather than defaulting to the longest board available.

Joist spacing, beam sizing, post spacing, footings, ledger attachment, and railing are all left out of this calculator entirely, and that's the strictest scope decision on this page. Every one of them is governed by the IRC's prescriptive deck-construction provisions, and getting one wrong isn't an over-order — it's a structural safety risk, up to and including a deck collapse. This page won't state a joist-spacing figure, not even as general context, because a number like that shows up everywhere in decking source material and doesn't belong here regardless of how commonly it's repeated. Get every one of those six from the current IRC and your local building department, not from a materials calculator.

Common mistakes

Ordering board count off nominal width alone, with no allowance for the gap beside each board. A board's real coverage is its width plus the gap, not its width alone — skipping the gap is the single most common way this calculator's math differs from a rough guess.
Averaging or guessing a gap between Trex's and TimberTech's published ranges instead of checking your own product's instructions. The two companies genuinely disagree because their products behave differently — a blended number matches neither.
Using the same spacing approach for wood and composite decking. Wood expands mainly across its width; composite expands mainly along its length — which is why width gaps matter more for wood and butt-joint gaps matter more for composite, not the reverse.
Installing pressure-treated lumber with the wrong gap for its moisture state. Wet PT installed tight can end up over-gapped once it dries and shrinks; dry PT installed tight has nowhere to expand. Ask your supplier whether your lumber is wet or kiln-dried before setting your gap.
Defaulting to the longest available board length without checking it against your deck's actual dimensions. Longer boards cut fewer joints but can leave more unusable drop when a run doesn't divide evenly into that length.
Assuming this calculator estimates joist spacing, beam sizing, post spacing, footings, ledger attachment, or railing. It doesn't, on purpose — all six are IRC-governed structural elements where a wrong number is a safety risk, not an over-order, and belong with the current code and your local building department.

Where the figures come from

Trex and TimberTech are cited for two different things. Their spacing pages back the gap ranges stated in this calculator's gap field and in the guidance below — real figures that genuinely disagree between the two manufacturers, not a single number this page picked. TimberTech's installation guide is cited for something stronger than a figure: it describes this exact calculation in its own instructions — "the actual width needs to incorporate both the deck board width and the spacing" — and its worked example uses a 5.5 in. board, which is where this calculator's board-width default comes from. There's no coverage-per-board figure to cite beyond that; it's always computed from your own board length, width, and gap.

FAQ

Why does the gap between deck boards affect how many boards I need?: Because a board's real coverage is its width plus the gap beside it, not its width alone. A 12 ft, 5.5 in. board at a 3/16 in. gap covers 5.69 sq ft; the same board at a 3/8 in. gap covers 5.875 sq ft — about 3.3% more. Across a whole deck, that difference moves your board count by roughly the same percentage.
What gap should I use between deck boards?: Whatever your manufacturer's installation instructions specify — this page won't guess, because real manufacturers genuinely disagree. Trex recommends 3/16 in. to 3/8 in. for its composite boards. TimberTech's guidance is typically 1/8 in. to 1/4 in. Both are correct for their own products; neither is a universal number.
Why do wood and composite decking need different spacing?: Because they expand and contract in different directions. Traditional wood moves primarily across its width, so the width-to-width gap does most of the work. Composite moves primarily along its length, which is why the gap at butt joints (where boards end) matters more for composite than it does for wood.
Should pressure-treated decking be installed with a gap?: It depends on whether the lumber is wet or dry when it's installed, a distinction worth checking with your supplier rather than assuming. Wet PT lumber is often installed tight, or nearly so, because it shrinks as it dries and creates its own gap over time. Dry PT lumber doesn't get that free shrinkage and needs a real gap left on purpose.
What board length should I use?: There's no single right answer — it's a tradeoff. Longer boards (16 or 20 ft, alongside the common 12 ft length) mean fewer butt joints and less offcut per foot, but a length that doesn't divide evenly into your deck's dimension can leave an unusable drop. Check your options against your deck's actual dimensions rather than defaulting to the longest board available.
Does this calculator cover joist spacing, footings, or railing?: No, deliberately. Joist spacing, beam sizing, post spacing, footings, ledger attachment, and railing are all governed by the IRC's prescriptive deck-construction provisions, where a wrong number is a structural safety risk, not just an over-order. This calculator counts deck boards only — get those six from the current IRC and your local building department.
Why does this calculator show results in linear feet as well as a board count?: Because some suppliers quote and sell decking by the linear foot rather than by the piece. Linear feet here is simply the board count multiplied by your chosen board length — the same order, in the units your supplier might actually ask for.