How this calculator works
Subtract repeated door and window areas from the wall or ceiling area. The exposed face width times usable length is installed coverage per board. Divide the net area with waste by that coverage and round up. Labour uses net installed area. This method assumes reusable offcuts; a row-by-row layout can require additional full lengths.
Formula
net area = gross area - doors - windows; coverage per board = usable length x exposed width; boards = ceil(net area x (1 + waste)/coverage); linear length = order area/exposed width.
Worked example
A 4 m by 3 m wall less a 1 m by 2 m door leaves 10 m2. Boards with 2 m length and 0.5 m exposed width cover 1 m2 each, so ten boards are needed before waste.
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Frequently asked questions
Why not use the advertised nominal width?
The lap and dressed profile reduce coverage. Use the manufacturer exposed-face or installed-coverage dimension.
Can I calculate a ceiling?
Yes. Treat the two project dimensions as ceiling length and width. Enter any unboarded openings consistently.
Do doors and windows share sizes?
Each group repeats one size. For different sizes, calculate separate sections or enter a measured net area with both opening counts set to zero.