Deck Material Calculator: Board Count, Joists and a Metric Cut List

Every deck calculator on the internet gives you a square footage and a price. This one gives you the thing you actually take to a supplier: a board count, a joist count and a cut list — worked from cover width rather than nominal board width, and checked against the joist spacing the board is rated for.

It is set up for our own co-extruded boards, so the stock lengths and the spacing limit are real numbers rather than placeholders. If you are buying somebody else's board, the method still holds — swap in their cover width and their maximum joist centre.

One note on names, because the search results are a mess of them. A deck calculator, a decking calculator, a deck board calculator, a deck material calculator, a deck materials calculator, a decking boards calculator, a material calculator for deck work, a deck material list calculator — these are all the same job: turn a deck size into a quantity of boards. This is that tool. What it is not is a decking pricing calculator, and there is a section below explaining why that is deliberate.

Why Cover Width, Not Board Width

This is where most material take-offs go wrong, and it is worth thirty seconds.

A 140 mm board does not cover 140 mm of deck. It covers 140 mm plus the gap you leave between boards for drainage and thermal movement — 5 mm on ours. So the working figure is a 145 mm cover width, and one square metre of deck takes 6.90 linear metres of board.

Take the quantity off the nominal 140 mm instead and you under-order by about 3.5 per cent. On a 12 × 16 ft deck that is barely a board. On a 400 m² commercial job it is a bundle and a half, and it turns up as a shortfall on the last day of the install.

The Joist Spacing That Breaks Most Deck Plans

The maximum joist centre for these boards is 350 mm, which is 13.78 in.

That single number rules out the American default. Sixteen inches on centre is 406.4 mm — over the limit by 56 mm, or 16 per cent. There is no version of the calculation where it works, and almost every free deck plan you will find online is drawn at 16 in because that is what timber decking takes.

Twelve inches on centre is 304.8 mm and sits comfortably inside the limit. So does 350 mm itself, which is the metric spacing most of our customers frame to.

Two qualifications worth stating. The 350 mm figure is for a straight lay under distributed load. Lay the boards diagonally and the effective span between supports grows by about 40 per cent, so the centres have to come in. Put a hot tub, a planter or anything else concentrated on the deck and it needs its own framing underneath regardless of what the general spacing is.

Board Lengths Are Metric. That Changes the Cut List.

Our boards come in 2.9, 3.6, 4.0 and 5.8 m. In American terms that is 9 ft 6 in, 11 ft 10 in, 13 ft 1 in and 19 ft 0 in.

Read that list again, because two entries are traps. There is no 12 ft board and no 16 ft board. 3.6 m is 11 ft 10 in — two inches short of twelve, which is exactly the length that a US plan will assume runs clean across a 12 ft deck. And 5.8 m is 19 ft, not 16 or 20.

The fix is not complicated, but it has to happen at the right moment: build the cut list off the metric lengths from the start rather than converting a US drawing afterwards. A 12 ft span planned around 3.6 m boards either takes a 4.0 m board with a 400 mm offcut, or a deliberate joint. Both are fine. Discovering it on site is not.

Worked Examples: Common Deck Sizes

Every figure below comes out of the calculator above, at 10 per cent waste and 350 mm joist centres. Board counts are shown for the two lengths most people order.

Joist counts assume the boards run along the first dimension, so the joists are spaced along that direction and each joist spans the second. Turn the boards ninety degrees and the joist count changes — which is worth checking on a rectangular deck, because the cheaper orientation is not always the one that looks better.

Why This Is a Material Calculator and Not a Deck Cost Estimator

Every second tool in this space is a deck cost estimator or a decking pricing calculator, and they all have the same problem: the number they produce is somebody else's price in somebody else's market on a date that has passed.

A quantity is a fact. 400 sq ft of deck needs 282 linear metres of board with a 10 per cent allowance, and that is true in Texas in January and in Queensland in July. A price is not a fact. It moves with the grade, the colour, the volume, the port, the freight rate and the week. Publishing one as though it were a fact is how buyers end up budgeting a job against a figure nobody will honour.

What is worth knowing about where deck cost actually goes, if you are budgeting rather than ordering:

• On a raised deck, the boards are rarely the biggest line. Post count, footing depth and stair construction usually are — and all three come from the plan, not from the area. Two decks of identical square footage can differ substantially if one sits 18 in off the ground and the other 48 in.
• Labour runs 35–50 per cent of installed cost in most US markets, and varies more by metro than the material does.
• Freight is a real line on an import order and falls away fastest as you approach a full container. This is why quantity — the thing this calculator gives you — is the input a supplier actually needs to price against.

So: take the board count from here, send it to whoever is quoting, and get a comparable per-unit figure back. If that is us, the form on this page reaches the sales team, which will review the profile, quantity and destination before preparing the quotation.

What This Calculator Deliberately Will Not Tell You

Beam spans, post sizes and footing depths are missing, and that is on purpose.

Those numbers come from prescriptive tables in the IRC and your local amendments, and they depend on your deck height, your loading, your soil and your frost line. A decking supplier publishing span tables is a supplier hoping you will not check which jurisdiction they were written for. Take them from your building department, where they are free and correct.

What a supplier can tell you, and should, is the cover width, the maximum joist centre, the stock lengths and the weight. Those are the four numbers this page is built on, and they are the four that decide the order.

Frequently Asked Questions

How much material do I need for a 12 × 12 deck?

144 sq ft, which is 13.4 m². At a 145 mm cover width that is 92 linear metres of board net, or about 101 with a 10 per cent waste allowance — 29 boards at 3.6 m, or 18 at 5.8 m. Add 12 joists at 350 mm centres, each spanning 12 ft. Board weight is about 300 kg, boards only.

How much is material for a 20 × 20 deck?

400 sq ft, or 37.2 m². That is 256 linear metres of board net, 282 with 10 per cent waste — 79 boards at 3.6 m or 49 at 5.8 m — plus 19 joists at 350 mm centres spanning 20 ft. Board weight is about 832 kg. For the money side, the deck surface is rarely the biggest line on a raised deck; post count, footing depth and stair construction usually are, and all three come from the plan rather than the area.

How many decking boards will I need?

Divide the deck area in square metres by the cover width, not the board width. For a 145 mm cover that is 6.90 linear metres of board per square metre. Then divide by the stock length you are ordering, and round up. The calculator above does this for all four of our lengths at once, because the cheapest answer is not always the longest board.

Is this a deck cost calculator?

No, and deliberately not. It gives you a material list — board counts, joist counts, linear metres and weight — which is the input a supplier needs in order to quote. A published price would be somebody else's rate in somebody else's market, and it would be out of date before you used it. See the section above on why.

What is the 3-4-5 rule for decking?

It is how you square a frame without a large square. Measure 3 units along one side from the corner, 4 units along the other, and if the diagonal between those two marks is exactly 5 units, the corner is 90 degrees. Any consistent unit works — 3, 4 and 5 feet on a small deck; 6, 8 and 10 feet on a large one, which is more accurate because the error scales down. Check it before you fix anything, because a frame that is out of square puts every board cut out by the same amount all the way across.

What joist spacing do composite boards need?

Whatever the board you are buying is rated for, in writing. Ours is a maximum of 350 mm — 13.78 in — so 12 in on centre is within spec and 16 in is not. Other manufacturers publish their own figures and they are not interchangeable, so do not carry a number across from one brand to another, and do not carry a timber framer's default across to a composite deck.

Does the calculator include joists, posts and footings?

Joists yes, as a count and a length, because the spacing is set by the board. Posts, beams and footings no — those come from your local code. See the section above on what this calculator will not tell you, and why.

How much does composite decking weigh?

Our main board is 3.25 kg per linear metre, which works out at about 22.4 kg per square metre of finished deck. That is the boards alone — joists, pedestals and fixings are extra. The figure matters on balconies and roof terraces, where the structure has a stated load allowance, and it matters for freight, where it decides how much fits in a container.

Working out a pool surround instead of a rectangle? The above ground pool deck plans carry the same arithmetic for curved decks, including the polygon framing geometry. For the boards themselves, see the WPC decking range; for laying them, the installation guide.

Buying for more than one deck — a contractor, a distributor or a dealer? Send us the total square metres and the destination port and we will price it per container. Samples are free; you cover the courier and we credit it to your first order.

Turn the Quantity Into a Project Cost

This calculator establishes board and substructure quantities. Next, use the composite decking cost calculator to add profile pricing, accessories, freight, labour and project-specific allowances without treating a generic online figure as a quotation.