Free NZ decking screw calculator for 90mm and 140mm boards. Set joist spacing and screws per joist, get boxes of 500 and 1000, plus boards and a cut list.
This calculator works out how many decking boards a deck needs, the lineal metres to price, 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 4 x 3 m deck in 140mm boards with 5mm gaps takes 21 rows across the 3 m width, each row a 4 m run. The cut list buys them as 21 boards of 4.2 m, with a 0.20 m offcut on each, because 4.2 m is the stock length that covers a 4 m run.
The price counts what you buy, not what you lay: 4.2 m a board, not 4 m. With CostMate's allowance for offcuts, the calculator shows 22 boards of 4.2 m, which is 92.4 lineal metres: the cut list's 21 and one spare.
With 140mm boards, 5mm gaps, joists at 450mm centres and 2 screws per board per joist, decking takes 30.7 screws per square metre, or 33.7 with CostMate's allowance for wastage.
The same boards and gaps take 34.5 at 400mm centres and 23.0 at 600mm centres.
Change the joist spacing or the screws per board per joist and the count follows. For a quick figure, multiply the one with the wastage allowance by the deck area and round up to whole screws. With the deck's length and width filled in, the calculator counts every board row at every joist from its cut list instead, which can come out a little higher, and shows the result in boxes of 500 and 1000.
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. The 5mm starting value is CostMate's allowance, change it to suit.
Boards and lineal metres are shown twice: net, and priced, in whole stock lengths with CostMate's allowance of 10% for wastage: offcuts, splits, and the board you reject on sight.
The priced figure counts the length you buy, not the length you lay. A 5 x 3 m deck is 21 rows of 5.0 m cut from 5.4 m lengths, so it is priced at 5.4 m a board: 22 lengths, 118.8 lineal metres. The 0.4 m offcut on each length counts toward CostMate's 10% allowance, and one spare length makes up the rest.
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 metre, times joists per metre, times the fixings per board you choose: then CostMate's allowance of 10% added, and converted into 500 and 1000 boxes.
When the cut list shows, the count follows it instead: board rows, times joists, times fixings per board, plus CostMate's 10% allowance. So the screw total and the cut list on the page always agree.
Joist spacing options are 400, 450 and 600mm centres. The board width used for the fixings count starts from the board width you set above.
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 CostMate's allowance, 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.