The same page in feet and inches
A deck board is a deck board, so unlike brick or mortar this genuinely is a unit change rather than a different set of objects. What differs is what the yard sells you and what the frame underneath is spaced at.
- 5/4 by 6 is the standard American deck board and it measures 1 inch by 5-1/2, under the same PS 20 standard that makes a 2x4 measure 1-1/2 by 3-1/2.
- 16 inches on center is the ordinary joist spacing, not 400mm.
- Boards come in even foot lengths, so 12, 16 and 20, which means an odd run wastes a whole foot per board before a single cut.
Composite is the one that needs care, and it is not about strength. Composite expands and contracts with temperature where wood moves with moisture, so the gap between boards depends on how long the run is and how warm it was when you fixed it. The tool sets a wider default and then tells you to read the manufacturer instructions, because that number is genuinely theirs and getting it wrong buckles a deck in July.
One number falls out of this that is worth seeing before you commit to a pattern. Bringing 16 inch centres in by the diagonal factor of 1.41 lands on 11.3 inches, which is effectively the 12 inch spacing composite makers ask for on a 45 degree lay anyway. So the cost of a diagonal deck is not the extra cuts, it is a third more joists.
The gap is part of what a board covers
A 144mm deck board is not 144mm of deck. Laid with a 5mm gap it occupies 149mm, and that is the number the row count comes off.
Across a 4 metre width that is 27 rows, not 28. Over a bigger deck it is several boards, and the error runs in the direction of buying timber you do not need and cannot take back.
Nobody counts the gap because it feels like a detail. It is about 3 per cent, and 3 per cent of a decking order is real money. The tool shows the coverage figure as a number rather than hiding it in the maths, and tells you what the other answer would have been.
The gap is not optional
Worth saying, because "why not just lay them tight and get more boards on" is a reasonable question.
Softwood moves with moisture. Boards laid tight cup, trap water at the joints and rot early, and there is nowhere for debris to fall through, so the deck stays wet after rain. Five millimetres is what stops all of that.
Fourteen joists, not thirteen
A 5 metre run at 400 centres. Divide and you get 12.5, round up to 13, and you are one short: because there is a joist at each end.
Exactly the same trap as fence posts and stud walls. The division counts the gaps, and the thing you are buying is the timbers between them plus one.
Diagonal decking costs more than the cuts
This is the one that surprises people, usually after the joists are already down.
Lay boards at 45 degrees and each board crosses the joists on the diagonal. The distance it spans between two joists is no longer the centres: it is the centres times 1.41.
Which means the joists have to come in to compensate: 400 centres becomes about 283, and that is a third more joists across the whole deck. Switch the pattern on the tool and watch the joist count move.
A diagonal deck is not "the same deck with more cuts". It is a different frame, and the extra timber usually costs more than the extra labour.
Waste on decking is worse than it looks
Ten per cent, and that is not generous.
Every row gets cut to length, and the offcut from one row is the wrong length for the next unless the deck happens to be exactly two boards wide. Unlike plasterboard, where an offcut usually finds a home somewhere, decking offcuts mostly become firewood.
If the deck is wider than a board, joints get staggered between rows, which matters a great deal for how it looks and hardly at all structurally, as long as every joint lands on a joist.
Joist size and span are not on this page
It does not size the joists, work out a span, or decide how the frame is supported. Those depend on what is underneath and what is going on top, and they are not questions a web page should answer.
This counts boards, joists and screws for a frame you have already decided on. Take the lengths to the cutting list and it will work out the stock to buy, including the saw kerf.
Common questions
How many deck boards do I need?
Divide the width of the deck by what one board actually covers, which is the board face plus one gap, then multiply by the length. A 16 by 12 foot deck in 5/4 by 6 with a 3/16 inch gap is 26 rows, because 144 inches over 5.6875 is 25.3 and you round up. Work it off the board width alone and you get 27, which is a whole extra row you do not need. That is the commonest error in the subject and it is an over-order, not a shortfall.
What is a 5/4 deck board actually?
One inch by 5-1/2 inches. Nominal 5/4 by 6 dresses down under the same US Department of Commerce standard, PS 20, that makes a 2x4 measure 1-1/2 by 3-1/2: nominal 5/4 becomes 1 inch and nominal 6 becomes 5-1/2. So the face you are laying is 5-1/2 wide whatever the yard calls it, and a 2x6 deck board has the same 5-1/2 face with half an inch more thickness under it.
What gap should I leave between deck boards?
For wood, three sixteenths of an inch is the usual starting point, on the reasoning that pressure treated lumber arrives wet and shrinks as it dries, so a tight joint today opens up anyway and a tight joint that stays tight traps water. Composite is a different problem: it moves with TEMPERATURE rather than moisture, so the gap depends on the length of the run and the air temperature on the day you fix it, and the number has to come from the manufacturer instructions rather than from a rule of thumb.
How much decking do I need, and how much decking will I need after waste?
Work it off coverage, not board width. A 144mm board laid with a 5mm gap covers 149mm, so a 4 metre width takes 27 rows rather than the 28 you get from dividing by 144. Over a big deck that is several boards of difference, and it is in the direction of buying timber you do not need.
How to work out how much decking I need
Area first, then coverage, then waste, and the middle step is the one that catches people. Length times width gives the deck area. Divide the width by the COVERAGE of a board rather than its width, because the gap between boards is part of what each one covers. Then add waste for cuts, and more if you are laying diagonally, because a diagonal board crosses the joists at 45 degrees and every board needs two angled cuts instead of none. The tool does all three and shows coverage separately so you can see where the difference comes from.
How many deck boards do I need?
Divide the width of the deck by the COVERAGE of a board rather than by its width, and the two are not the same number. A 144mm board laid with a 5mm gap covers 149mm, so a 4m width is 27 rows and not 28. That is about three per cent, it always falls in the direction of buying timber you cannot take back, and it is the single commonest reason a decking order comes back over. The tool shows coverage as its own figure rather than burying it in the arithmetic.
What gap should I leave between deck boards?
Around 5mm is usual for softwood. It is not optional: timber moves with moisture, and boards laid tight cup, trap water at the joints and rot early. Wider boards want a slightly bigger gap, and the manufacturer’s instructions matter for composite.
How many joists do I need?
Run length divided by centres, plus one. A 5 metre run at 400 centres is fourteen joists, not thirteen: one at each end and twelve between. Same arithmetic as fence posts and studs, and it catches people just as often.
Does laying decking diagonally cost more?
Yes, and by more than the extra cuts. A board laid at 45 degrees crosses the joists diagonally, so its span between them goes up by a factor of 1.41. The joist centres have to come in to compensate: 400 becomes about 283, which is a third more joists. Nobody mentions that when they suggest a diagonal deck because it looks better.
How much waste should I allow on decking?
Ten per cent is sensible. Every row needs cutting to length and the offcut is the wrong length for the next row, so decking waste is worse than it looks, unlike, say, plasterboard where an offcut often finds a home.
What size joists and what span?
Not a question this tool answers, and not one a website should. It depends on what is under the deck, how it is supported, and what is going on it. This works out quantities for a frame you have decided on.