Free NZ decking calculator. Board count, joists, bearers, and fixings for timber decks.
This calculator works out how many decking boards a deck needs, the lineal metres to order, the fixings to go with them, and a cut list showing what to buy and where each board gets cut. It is a take-off tool: it counts material, it does not design the deck.
A decking board takes up its own width plus the gap beside it. So the number of rows across a deck is the deck width divided by the board width plus the gap, rounded up — not the deck width divided by the board width.
The difference is not trivial. A 140mm board with a 5mm gap covers 145mm, so a 3.0m wide deck takes 21 rows. Divide by 140mm instead and you get 22 — one row of boards you did not need, on every deck you quote.
Board widths here are 90mm and 140mm, thicknesses 19mm and 25mm, and the gap defaults to 5mm and can be set from 0 to 20mm.
Every quantity is shown twice: net, and with 10% added for wastage — offcuts, splits, and the board you reject on sight.
The lineal-metre figure is derived from the rounded-up board count, not from the raw one. That matters when you hand the number to a merchant: 24 boards at 5m is 120 lineal metres. Order 115.5 lineal metres instead and you are asking for 23.1 boards.
Quantities are rounded to six decimal places before rounding up, so floating-point arithmetic cannot add a phantom extra board to an order that came out even.
Screws are counted as boards per square metre, times joists per metre, times the fixings per board you choose — then 10% added, and converted into 500 and 1000 boxes.
Joist spacing options are 400, 450 and 600mm centres. The board width used for the fixings count follows the board width you set above, so the two sections cannot disagree.
The cut list turns the same deck into an order: how many stock lengths to buy, what to cut from each one, and what is left over. Where a run is longer than any single board, it is joined, and every join is placed on a joist centre.
It buys from stock lengths of 1.8m to 6.0m in 0.6m steps. Those lengths are an assumption, not a merchant quote — confirm what your supplier actually carries before you order, because a length they do not stock changes the count.
The cut list covers decking boards and joists. It does not plan bearers, piles or blocking.
Spans, fixings and barrier heights are design decisions. Check them against your plans and NZ Building Code B1 (Structure), and confirm stock lengths with your merchant, before you order or build.