Construction & trade New Zealand
Decking Calculator NZ
Enter the deck size, the board profile you are buying and the joist centres. The calculator returns board count, lineal metres, joist count and fixings — with a wastage allowance you can set for the layout you are actually building.
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Along the direction the boards run
Across the boards
3–6 mm for kiln-dried, less for wet-treated
10% square, 15% with angles or a border
Boards to order
—Rows across
—
Lineal metres
—
Joists
—
Fixings (2 per crossing)
—
How many decking boards do I need?
Divide the deck area by the area one board covers, including the gap between boards. A 5 m × 4 m deck using 140 mm boards with a 5 mm gap covers 0.145 m per board across, so a 4 m span takes 28 boards. Add 10% for cuts and defects before ordering.
Joist centres and what they depend on
Joist spacing is not a preference. It follows from the board thickness and the timber, because the board has to span between joists without deflecting underfoot.
Closer centres let thinner boards work and give a firmer feel. Wider centres use less framing timber and need thicker or stiffer decking. Four hundred and fifty millimetres is the common domestic figure for typical decking profiles, with 400 used for thinner boards and diagonal layouts.
A diagonal lay is the case people miss: laying boards at 45 degrees increases the effective span between joists by about 40 per cent, so a spacing that works for a straight lay may not for a diagonal one.
Span tables for specific board profiles and timber treatments are published in NZS 3604 and in supplier literature, and are not reproduced here. Take the profile and the treatment you are actually buying to the merchant and get the figure for it.
The span your specific board and timber can carry comes from the supplier's span tables or the NZ Building Code documents, not from a general rule. Take the profile and the treatment to the merchant and get the figure for what you are actually buying.
Wastage, and why it is never zero
Three things eat into an order and none of them is avoidable.
Cuts at the ends of runs leave an offcut that is usually too short to use elsewhere. On a deck whose length does not divide neatly into the board lengths stocked, this is the largest single loss.
Defects are the second. Every pack has a board or two with a split end, a large loose knot or a bad bow, and you will not want it in the middle of a deck.
And the layout is the third. A rectangular deck laid straight is the cheap case at around 10 per cent. A picture-frame border, a diagonal lay or an angled edge pushes it to 15 and sometimes past it.
Order the whole quantity at once. Timber colour and treatment shade vary between packs, and a top-up delivery weeks later is visible in the finished deck.
How this is calculated
A deck is rows of boards laid across joists, and both counts come from a division and a rounding up.
board coverage = (board width + gap) ÷ 1,000 metresrows across = deck width ÷ board coverage, rounded uplineal metres = rows × deck length × (1 + wastage %)boards to order = lineal metres ÷ board length sold, rounded upjoists = (deck length ÷ joist centres) + 1, rounded upfixings = rows × joists × 2
The gap is the part that gets left out and it changes the count. Five millimetres on a 140 millimetre board is 3.4 per cent of the coverage, which over a four metre width is most of a board's worth of difference.
The gap itself is not optional. Timber moves with moisture, and boards laid tight will cup or lift as they dry. Kiln-dried decking is normally laid with a 3 to 6 millimetre gap; wet-treated timber is laid closer because it shrinks as it dries and opens its own gap over the first summer.
A worked example: a 5 by 4 metre deck
A 5 metre by 4 metre deck in 140 mm boards with a 5 mm gap, 5.4 metre lengths, joists at 450 mm centres and 10% wastage.
- Board coverage — 140 + 5 mm0.145 m
- Rows across — 4 ÷ 0.14528 rows
- Lineal metres — 28 × 5140 lm
- Add 10% wastage154 lm
- Boards at 5.4 m — 154 ÷ 5.429 boards
- Joists — (5 ÷ 0.45) + 113 joists
About 154 lineal metres — 29 boards at 5.4 metres, over 13 joists at 450 centres.
Ordering in 5.4 metre lengths for a 5 metre run leaves 400 millimetres of offcut per board. Where the run suits it, a length that divides more cleanly wastes less, which is worth checking against what the merchant stocks.
What this calculator does not do
- Assumes a rectangular deck with boards running the full length
- Does not size joists, bearers or piles — those come from span tables for your timber
- Excludes framing, stairs, balustrade and substructure timber
- Fixings assume two per board-joist crossing; hidden fastening systems differ
Common questions
How many decking boards do I need per square metre?
Divide one metre by the board coverage including the gap. A 140 mm board with a 5 mm gap covers 0.145 m, so about 6.9 rows per metre of width, and 6.9 lineal metres of board per square metre of deck.
What gap should I leave between decking boards?
Three to six millimetres for kiln-dried timber. Wet-treated timber is laid tighter because it shrinks as it dries and opens its own gap over the first summer.
What joist spacing is standard for a deck in New Zealand?
Four hundred and fifty millimetres is common for typical domestic decking profiles, with 400 for thinner boards or a diagonal lay. The correct figure for your timber comes from the supplier's span tables rather than a general rule.
How much wastage should I allow for decking?
Ten per cent for a rectangular deck laid straight, 15 per cent or more for a diagonal lay, a border or angled edges. Order it all in one delivery so the timber matches.
Should boards run along or across the deck?
Across the joists, always — the board spans between them. Which way the deck reads visually is then decided by which way the joists run, so that decision belongs at the framing stage.
Where these numbers come from
- Building Performance — New Zealand Building Code Timber framing and decking guidance for NZ construction
- Standards New Zealand NZS timber framing standards referenced by span tables
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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