Formula
Boards use the same rule as plasterboard: area times one plus waste, divided by the area of one board, rounded up.
Rolls are the area times one plus waste, divided by the coverage printed for one roll, rounded up.
Worked example
20 m² with 5% waste on 2400 × 1200 mm boards is 7.3 boards, rounded up to 8.
Assumptions
Boards start at 2400 × 1200 mm. Waste starts at 5% for both modes.
The roll example uses 8.0 m² of coverage. That number is an example, not a brand.
Nothing here says whether the layer is warm enough. Pick the thickness before you count the pieces.
Checked
Checked 10 November 2026. The Celotex handy guide lists boards at 1200 × 2400 mm. The 8 m² roll coverage is still an example figure, not a brand’s printed cover.