Measuring a deck so the order matches the ground
A deck is a rectangle (or a couple of rectangles pushed together) until it isn't, and most of the surprises in a decking order come from treating it as one when it actually has a step, a wraparound corner or an angled edge against an existing path. For this calculator, measure the simplest rectangle that covers the main deck area, run it through, and if there's a step-down section, an L-shaped return or an angled bite taken out of one corner, run that separately and add the two results together rather than trying to force an irregular shape into one set of dimensions. Measure to the outside edge of the frame, not to where the boards happen to overhang, since overhang is a finishing choice, not part of the structural area.
Everything below works off two numbers you feed it and two you already know: the deck's length and width, the board you've chosen (or are pricing up), and how you want to fix it down. Change any of them and the board count, joist count and screw count all move together, which is the point. A board swap partway through a quote shouldn't mean redoing the whole take-off by hand.
Why the gap between boards counts as covering width
The number that catches people out first is covering width. It is not the board's own face width, it's the face width plus the gap deliberately left between one board and the next. A 90mm board laid with a 5mm gap on each side effectively takes up around 95 to 100mm of deck surface once its neighbour is in place, not 90mm, and across a whole deck that difference adds up to a real number of extra lineal metres if it's ignored. The gap itself isn't decorative. Timber decking moves with moisture, swelling and shrinking across the seasons, and the gap gives it somewhere to go without boards cupping against each other or a board splitting because it had nowhere to expand into. Composite boards need a gap for the same reason, thermal movement rather than moisture, usually specified by the manufacturer rather than left to the installer's judgement. Whatever gap you're running, add it to the board's face width before entering the covering width above. That single number is doing more work in this calculator than any other input.
Board length and the waste a straight run still generates
Boards come in fixed lengths, and a deck almost never divides evenly into them. A 6m deck run against 5.4m boards means every second or third row needs a join, an offcut, or a board bought longer than needed and trimmed, and that's before accounting for boards that snap, split at a screw hole, or get cut wrong on site. The wastage allowance above sits on top of the straight lineal-metres figure for exactly that: a straightforward rectangular deck with clean 90 degree corners needs less of it, a deck with a diagonal border pattern, multiple returns, or picture-framing around the edge needs noticeably more, because every mitred corner and every board that has to be cut to fit an angle produces an offcut that can't be reused on the next row. There's no single right figure here. It's a judgement about the specific deck's layout, which is why it's a field you set rather than a number this tool assumes on your behalf.
Joist spacing is a span-table decision, not a calculator guess
Joist spacing looks like the kind of number a calculator should just know, and that's exactly why it's worth being upfront about what this one doesn't do. Joist spacing isn't a fixed figure. It depends on the joist's own size and species, how far the joist spans between bearers, the decking board's thickness and species, and the load the deck is designed to carry, and all of that is set out in the span tables published by the timber or engineered-joist manufacturer, or worked out by a builder or engineer against AS 1684 for the specific combination on your job. Reproducing those span figures here would mean asserting a number this tool has no way of knowing is right for your joist, your span or your decking product, so it isn't going to. Enter the spacing your span table or your builder has specified, and this calculator will tell you how many joists that spacing needs across the deck's length and, from there, how many screw fixings the job comes to. Get the spacing itself from the people who publish it for the timber you're actually using.
Why the screw count is always bigger than people expect
Screw count is the number that most often makes someone stop and double-check the maths, because it isn't one screw per board, it's one fixing point at every single crossing of a board over a joist, and a deck has a lot of those crossings. A 6m long deck crosses more joists than most people picture at a glance, often well over a dozen along its length depending on the spacing your span table specifies, and if every board is fixed with two screws at every one of those crossings, a deck that only needed, say, twenty five boards to cover can easily need several hundred screws, sometimes close to a thousand. That number is correct, not inflated. It's simply what fixing every board securely to every joist it crosses actually adds up to once you count it properly instead of estimating by eye. Buy screws in bulk boxes rather than by the handful, and buy slightly more than the raw figure here, since a dropped screw in the grass or a stripped head on a knot in the timber is a normal part of the job, not a sign something went wrong.
Hidden fixing clip systems change this arithmetic. Some clip into a grooved edge on the board rather than screwing through the face, and the fixing count for those follows the clip manufacturer's own spacing, not a per-board screw count. If you're running a hidden-fix product, use its own published fixing rate rather than the screws-per-board field above, which assumes a conventional face-fixed board.
Timber and composite take different inputs, not a different tool
This calculator doesn't care whether the board is hardwood, treated pine or a composite product, because the geometry is the same either way, area covered by lineal metres of board at a given covering width. What changes between the three is the numbers you feed it, not the maths itself. A composite board often runs wider and with a smaller gap than a hardwood board of similar coverage, which changes the covering width input. Composite also tends to specify its own joist spacing (frequently tighter than an equivalent timber board, because composite decking flexes differently under load and heat), published by that specific manufacturer for that specific profile, not a general-purpose figure that carries across brands. If you're comparing a timber and a composite option for the same deck, it's worth running this calculator twice, once with each product's own covering width, board length and joist spacing, rather than assuming the board count and joist count will land in the same place. They usually don't.
What sits under the boards
Everything above deals with the deck surface. Underneath it, joists sit on bearers, and bearers sit on posts and footings, and that substructure carries the same design-table logic as the joist spacing itself: post spacing, bearer size and footing depth all come off the same kind of span tables and, for anything load-bearing or attached to the house, usually a council building permit. A deck more than a step or two off the ground, or one attached to the dwelling rather than free-standing, is very often not a job you can just build to a calculator's output and start digging. Ground clearance for airflow under the boards matters too, both for the timber's own longevity and to stop the space becoming a damp, unventilated cavity against the house. None of that changes the board, joist or screw counts above, but it's worth having sorted, permit included where one applies, before the first bag of concrete goes in a footing hole.
Fasteners, and the one thing worth checking before you buy
Whatever fastener you're running, it needs to suit both the decking timber and whatever it's screwing into. Tannin-rich hardwoods like merbau will stain and corrode an unsuitable fastener within a season, which is why decking screws are almost always a coated or stainless product rather than a bare zinc-plated general-purpose screw. It's a cheap thing to get right at the point of ordering and an expensive thing to fix once black staining has already run down the face of every board on a finished deck.
Where this fits with the rest of the job
Decking sits alongside a run of other outdoor trade work: fencing, pergolas, sometimes a new roof over the same area, and the calculators on this site cover several of those jobs the same way, converting a measurement into an actual material count rather than leaving you to guess a wastage figure or a coverage rate from memory. If you're a carpenter or a decking specialist rather than a homeowner working out a job, see how we build websites for carpentry and decking businesses that turn a search like this one into an actual enquiry.