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.
Deck Material Calculator
Enter the deck size. Everything below updates as you type.
Direction the boards run
Direction the joists run
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.
What goes wrong if you ignore it. Nothing dramatic, which is the problem. The deck does not fail inspection and it does not collapse. It flexes underfoot — a soft bounce between joists that you feel on every step. It does not settle down over time, and the only fix is to pull the boards up and add joists. People live with it for years and never work out what the cause was.
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.
| Stock length | Imperial | Runs clean across |
|---|---|---|
| 2.9 m | 9 ft 6 in | A 9 ft deck, or 8 ft with trim |
| 3.6 m | 11 ft 10 in | An 11 ft deck — not a 12 ft one |
| 4.0 m | 13 ft 1 in | 12 ft and 13 ft decks |
| 5.8 m | 19 ft 0 in | 18 ft and 19 ft decks |
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.
| Deck | Area | Decking +10% | 3.6 m boards | 5.8 m boards | Joists | Board weight |
|---|---|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft · 9.3 m² | 70 lin m | 20 | 13 | 10 | 208 kg |
| 12 × 12 ft | 144 sq ft · 13.4 m² | 101 lin m | 29 | 18 | 12 | 300 kg |
| 12 × 16 ft | 192 sq ft · 17.8 m² | 135 lin m | 38 | 24 | 12 | 400 kg |
| 12 × 24 ft | 288 sq ft · 26.8 m² | 203 lin m | 57 | 35 | 12 | 599 kg |
| 14 × 20 ft | 280 sq ft · 26.0 m² | 197 lin m | 55 | 35 | 14 | 583 kg |
| 16 × 16 ft | 256 sq ft · 23.8 m² | 180 lin m | 51 | 32 | 15 | 533 kg |
| 16 × 20 ft | 320 sq ft · 29.7 m² | 226 lin m | 63 | 39 | 15 | 666 kg |
| 20 × 20 ft | 400 sq ft · 37.2 m² | 282 lin m | 79 | 49 | 19 | 832 kg |
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.
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.
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 composite 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.
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