Formula
Each board covers its face width plus the gap. Rows are the width across the boards divided by that cover, rounded up.
Lengths along the run are the board run divided by the stock length, rounded up. Net boards are rows times those lengths. Waste applies only to that board count, then it is rounded up.
Joist lines are the board run divided by the centre spacing, rounded up, plus one. Each line is split into stock lengths across the width. Joists get no waste allowance.
Worked example
Cover width is 0.150 m. 3.6 m needs 24 rows, and 4.8 m needs 2 stock lengths: 48 boards net.
With 10% waste that is 52.8 boards, rounded up to 53. Joists: 13 lines, 13 lengths of 4.8 m. No waste is added to the joists.
Assumptions
UK defaults are a 145 mm face, with 120 mm also listed, and 3.6 m stock. Australia defaults are a 90 mm face, with 140 mm also listed, and 5.4 m stock.
The gap starts at 5 mm. Joist centres start at 400 mm and the joist stock at 4.8 m, in both countries.
Switching country changes the board face and the stock length. It leaves the deck size, gap, waste and joist numbers as you set them.
Checked
Checked 10 November 2026. B&Q lists a timber deck board at 3.6 m × 145 mm. A Bunnings merbau guide uses 90 mm and 140 mm faces. That guide does not list a 5.4 m stock length, so the Australian length stays a preset you can change.