About the Insulation Thickness Calculator
Insulation thickness needed to reach a target U-value or R-value for a given material.
Also known as: how thick insulation, insulation depth calculator, loft insulation thickness, insulation needed for u value.
How to use this calculator
- Enter target, insulation, existing r-value of the structure and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
thickness = (target R − existing R) × conductivity
Worked example
Using the values pre-filled in the calculator above:
- Target: U-value
- Target U-value = 0.18 W/(m²·K)
- Insulation: Mineral wool (0.035)
- Existing R-value of the structure = 0.5 m²·K/W
the calculator returns:
- Insulation thickness: 176.9 mm
- Next standard thickness: 200 mm
- R-value needed from the insulation: 5.0556 m²·K/W
- Total R-value achieved: 6.2143 m²·K/W