Insulation rolls Calculator: measuring notes and method
Match width to the spaces
Enter the actual roll width and length for the selected thickness. A total-area result can understate cuts when roll width does not suit the cavity widths.
Check the installed fit
Area coverage alone does not establish a good fit. Check gaps, compression and cuts against the chosen product’s installation detail.
Formula
Effective roll area = (roll width − side overlap) × roll length. Rolls = area with waste ÷ effective coverage, 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
- Roll width
- 1.2 m
- Roll length
- 8 m
- Side overlap
- 0 mm
- Waste allowance
- 10 %
Rolls to buy: 3
Imperial worked example
- Total surface area
- 258.3339 ft²
- Roll width
- 3.937 ft
- Roll length
- 26.2467 ft
- Side overlap
- 0 in
- Waste allowance
- 10 %
Rolls to buy: 3
Follow the calculation
Lengths in this working are in metres.
- Effective width = 1.2 − 0 = 1.2 m.
- Effective area per roll = 1.2 × 8 = 9.6 m².
- Divide 26.4 m² including allowance by effective roll area and round up.
Reviewed 19 October 2026. How calculations are checked · Report a correction
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