Rigid insulation boards Calculator: measuring notes and method
Separate layers and thicknesses
Calculate each board layer independently where thickness or coverage changes. For identical layers you can combine their areas, but name the project item clearly so a second layer is not forgotten.
Check edge and cutting details
Board dimensions, rebated edges and usable coverage may differ. Confirm the area the product actually covers and account separately for perimeter cuts, joints and fastening requirements.
Formula
Sheets = (area − openings) × (1 + waste ÷ 100) ÷ (sheet length × sheet width), rounded up.
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Total surface area
- 24 m²
- Openings / excluded area
- 0 m²
- Sheet length
- 2.4 m
- Sheet width
- 1.2 m
- Waste allowance
- 10 %
Sheets to buy: 10
Imperial worked example
- Total surface area
- 258.3339 ft²
- Openings / excluded area
- 0 ft²
- Sheet length
- 7.874 ft
- Sheet width
- 3.937 ft
- Waste allowance
- 10 %
Sheets to buy: 10
Follow the calculation
Lengths in this working are in metres.
- Net area = 24 − 0 = 24 m².
- Area per sheet = 2.4 × 1.2 = 2.88 m².
- Sheets = 24 × 1.1 ÷ 2.88, rounded up. Check the cutting layout separately.
Reviewed 19 October 2026. How calculations are checked · Report a correction