Decking Calculator

A 16 × 12 ft deck comes to 28 boards and 448 linear feet

The board count comes off the deck’s width, not its length. The estimate assumes the decking runs across the deck, perpendicular to the joists, with each board spanning the full length — a straight, single-direction layout, not diagonal or picture-frame. So the count is set by how many board widths fit across the deck’s width, and each board is bought at the deck’s length. The calculator divides the deck width by the board width plus the gap you enter, then applies your waste factor and rounds up.

Take a 16 × 12 ft deck with 5.5-inch boards, a 1/4-inch gap and 10% waste. Area: 16 × 12 = 192 sq ft. Boards: ⌈144 ÷ 5.75 × 1.10⌉ = 28 boards, each 16 ft long — 448 linear ft in total. The rounding is always upward — a fractional board is still a board you have to buy, so the 27.5 the arithmetic returns becomes 28 — which is why the gap and the waste factor can each tip the order by a whole board.

That 448 is the Total Decking Length: the board count multiplied by the deck’s length, the total run of decking to order, in linear feet or linear metres. Decking is priced and stocked by length, not just by area, so this is the figure to hand a supplier once you know your deck’s square footage or square metres. Order the boards at the deck’s length so each one runs uninterrupted across the joists; the waste factor covers end trims and the occasional crooked or split board.

People often assume the board count covers a fancy layout. It does not. Diagonal decking and picture-frame borders need noticeably more material and a higher waste factor than the straight, full-length runs this estimate assumes.

The same deck in metric: 20 m², 31 boards, 155 linear metres

Switch “Measurement Units” to Metric and every field converts automatically to metres (deck length and width) or millimetres (board width, gap, and joist spacing), keeping the same real-world quantity you already entered. All the outputs — boards, area, joists, fasteners, cost — use the same formulas either way: switching between Imperial and Metric converts every entered value and then runs exactly the same arithmetic, just in feet and inches or metres and millimetres.

Run that same deck through in metric — 5 × 4 m, 140 mm boards, 6 mm gaps, 400 mm centres — and you get a 20 m² deck needing 31 boards, or 155 linear metres of decking, on 14 joists and roughly 806 screws. Switch to hidden clips and the fastener count is roughly half.

What does break the numbers is mixing units mid-project. Entering some measurements in feet and others in metres without switching the toggle is an easy way to throw off every downstream number, so pick one system before you start typing and stay in it.

Composite decking does not automatically need tighter joists than timber

Joist count divides the deck length by the joist spacing and adds one, because a framed bay needs a joist on both ends. On the 16 × 12 ft deck at 16-inch centres: ⌈192 ÷ 16⌉ + 1 = 13 joists.

What size those joists have to be is a span question, and this calculator does not answer it — it does not check joist span against load. Table 2 of the American Wood Council’s DCA 6 deck guide gives these maximum joist spans at 16 in (400 mm) centres, varying by species: 2×8 spans 10 ft 7 in to 11 ft 10 in (3.2–3.6 m). 2×10 spans 13 ft 0 in to 14 ft 0 in (4.0–4.3 m). 2×12 spans 15 ft 1 in to 16 ft 6 in (4.6–5.0 m). The top of each range is Southern Pine and the bottom is redwood or western cedar, and every span assumes 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service conditions, and L/360 deflection.

Composite boards are governed by the board maker instead — but not in the direction most people expect. Composite sets its own joist-spacing limit rather than always needing a tighter one: TimberTech and Trex both cap standard perpendicular decking at 16 in (400 mm) on centre, the same as timber, and TimberTech tightens that to 12 in (300 mm) only for angled or commercial decks. The expensive error runs the other way — choosing composite decking and keeping 16 in (400 mm) joist spacing when that particular manufacturer requires 12 in (300 mm). Always check your product’s spec before locking in joist layout.

It is worth pricing that tighter spacing before you commit to it. Take the same 16 × 12 ft deck from 16 in centres to 12 in and the joist count goes from 13 to ⌈192 ÷ 12⌉ + 1 = 17 joists, while every board now crosses 16 joists instead of 12, so the screw count climbs from 672 to 896. Four extra joists and 224 extra screws is a real cost — but an undersupported composite board sags between joists, so follow the spec rather than the budget.

That is also why this is a generic calculator and not any retailer’s own tool. For composite boards — Trex, Wickes, B&Q, Bunnings or anyone else — enter your board’s actual width and gap, and switch joist spacing to whatever your product specifies. Then cross-check board width, spacing, and fastener spec against that specific product’s installation guide before ordering.

672 screws on that same deck, or roughly half with hidden clips

Fasteners are counted wherever a board crosses a joist, multiplied across every board — which is why the fastener total grows fast on a big deck. Face-screwed, that is two screws per crossing: 12 crossings × 2 × 28 boards = 672 screws. Select “Hidden clips/fasteners” and it falls to roughly one clip per crossing, since clips fasten through the board’s grooved edge instead of its face.

Two fasteners per joist is the published rule in AWC DCA 6 and in both the TimberTech and Trex installation guides. The one-clip-per-crossing ratio is a reasonable approximation, not a manufacturer spec — hidden-fastener systems vary by brand, so follow your clip manufacturer’s own coverage chart for the exact count.

Buy fasteners by the box rather than counting them out — the total tells you roughly how many boxes. And buy the right metal: use hot-dipped galvanized or stainless fasteners rated for treated lumber, because ordinary electroplated screws corrode fast where treated wood stays damp. Either way this is the largest count the tool produces, and the one most often bought short: a 100-screw box does not go far against a 672-screw total.

The 5–8 mm gap between boards is what keeps them from cupping

The UK’s Timber Decking & Cladding Association specifies roughly 5–8 mm (about 3/16–5/16 in) between boards for drainage, ventilation and seasonal movement; this calculator defaults to 1/4 in (about 6 mm), inside that range. Composite boards often specify their own exact gap — TimberTech requires a 1/8 in minimum to 3/16 in maximum side to side — so confirm the figure against your product’s spec sheet before you lock in your layout.

The gap is a quantity input, not just a detail. Narrow the same deck from a 1/4 in gap to that 1/8 in composite minimum and each board occupies 5.625 in rather than 5.75 in, which pushes ⌈144 ÷ 5.625 × 1.10⌉ to 29 boards instead of 28 — one more board and 24 more screws. Widen it to 5/16 in and you are back to 28.

A frequent error is butting boards tight with no gap at all; without drainage space, water sits and the boards cup and rot. The rest of the anti-warping routine is just as cheap: pre-drill to prevent splitting, seal wood end cuts with an end-grain sealer, and apply a penetrating sealer annually for pressure-treated timber. Let kiln-dried treated boards acclimate on site as well, so they shrink before, not after, you fasten them.

Plan the board layout so seams, if any, land centered on a joist, and stagger them between rows for strength and looks. A starter board snapped to a chalk line and checked for square keeps the whole field running straight.

$696.64 of boards and screws, before delivery, tax or labor

Enter a price per board and, optionally, a price per fastener, and the estimated material cost is shown alongside the quantities. Leave either at 0 to skip it. At $22 a board and $0.12 a screw, the 16 × 12 ft deck works out at 28 × $22 + 672 × $0.12 = $696.64, and that cost line is a starting budget for boards and fasteners together. The estimate is materials only; it does not include delivery, tax, or installation labor.

The waste factor is the one input in that total that is a planning allowance rather than a sourced figure. The 10% default covers end cuts and the occasional defective board; increase it for a complex layout or hand-picked material. On this deck it is worth exactly two boards: at 0% waste the count drops from 28 to 26, about $44 of timber held back against bad cuts.

Switching the fastener type to hidden clips halves the count to 336 and takes $40.32 off that same total at $0.12 apiece — but clips cost more per piece than screws, so enter their real price instead of assuming the saving.

Board choice then moves the per-board price far more than the count does. Most durable: ipe, teak, and tigerwood (30+ year lifespan but expensive). Great value: pressure-treated pine (15–20 years with maintenance). Naturally rot-resistant: cedar or redwood (15–20 years). Low-maintenance composite: PVC-core composite (25+ year warranty, no staining).

Where the estimate stops: posts, beams, the ledger and the permit

This tool turns a deck’s footprint into the quantities you buy by the piece: how many decking boards to order at the deck’s length, how many floor joists to frame the surface, and roughly how many screws or hidden clips to fasten it all down — plus an optional material cost, for a simple rectangular deck in timber or composite. It is meant for homeowners building a ground-level or low deck and for anyone pricing materials before a trip to the lumber yard, whichever unit system or fastening method they plan to use.

Use the joist count for the surface framing only. What it does not include is the frame under that surface. The tool covers the decking, the joists, and the fasteners; it does not size posts, beams, footings, joist hangers, or the ledger connection — the parts of a deck most likely to fail if undersized. Those are structural elements whose size and spacing depend on the deck’s height, span, and your soil and load, so they cannot be reduced to a single formula. Post and beam sizing in particular needs a structural engineer for decks over 30 inches (about 760 mm) off the ground or over 200 sq ft (about 18.5 m²). Use the board and joist counts for your shopping list, then size the frame from a span table or with a contractor — treat that as a separate step, not part of these numbers.

Permit requirements vary by jurisdiction, but most municipalities require one for decks over 30 inches (about 760 mm) off the ground, attached to the house, or over roughly 200 sq ft (about 18.5 m²), and those decks generally need an inspection as well. Permit fees typically run $50–$500. An unpermitted deck can cause problems at home sale and may void homeowner’s insurance for deck-related claims. Confirm framing sizes, the ledger attachment, and footing requirements with a licensed contractor or structural engineer, and check local code before building.

One product the name does not cover: roof decking. “Roof decking” (or roof sheathing) is the plywood or OSB layer under roofing material — a completely different product from the outdoor walking-surface decking this calculator covers. If you are sizing sheathing panels for a roof, you want a roofing or sheathing calculator instead.

Sources and methodology

Board gap, joist spacing and fastener counts follow published industry and manufacturer specifications rather than rules of thumb. Joist spans and the two-fasteners-per-joist rule come from the American Wood Council’s DCA 6 deck guide; composite joist spacing and gapping come from the TimberTech and Trex installation guides, which state the same 16 in on-centre limit and the same two-screws-per-joist rule independently of each other; the timber board gap follows the UK Timber Decking & Cladding Association specification. Waste factor is a planning allowance, not a sourced figure. Calculator Matters is not affiliated with any source listed. Links open in a new tab.

• American Wood Council — DCA 6 — Prescriptive Residential Wood Deck Construction Guide, 2015 IRC (Table 2 maximum joist spans at 12, 16 and 24 in on centre; DECKING REQUIREMENTS: attach decking to each joist with 2-8d threaded nails or 2-#8 screws)
• TimberTech (AZEK) — Composite Decking Installation Guide (joist spacing must never exceed 16 in on centre, tightening to 12 in for angled or commercial decks; two screws per joist; a 1/8 in minimum to 3/16 in maximum side-to-side board gap)
• Trex — 2024 Decking Installation Guide (independently confirms the same three figures: a 16 in / 406 mm on-centre span chart for 1 in boards, a 3/16 in width-to-width gap, and two screws per joist)
• Timber Decking & Cladding Association (TDCA) — Code of Practice: Raised timber deck structures on new homes, 6th ed. 2018, clause 3.9.1 Surface drainage (a gap of 5 mm minimum, 8 mm maximum shall be left between each board, for drainage, ventilation and seasonal movement — the range the 1/4 in / 6 mm default sits inside)

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