Insulation R-Value Calculator: measuring notes and method
Convert thickness before dividing
For an illustrative material 100 mm thick with conductivity 0.04 W/(m·K), use 0.1 ÷ 0.04 = 2.5 m²·K/W. Leaving thickness as 100 would produce an answer one thousand times too large.
Use the declared conductivity
Select conductivity for the specific product and stated conditions. Do not borrow the value from a different insulation material just because the thickness matches.
Formula
R = thickness in metres ÷ thermal conductivity. US R-value = SI R-value ÷ 0.1761101838.
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
- Layer thickness
- 100 mm
- Thermal conductivity
- 0.04 W/(m·K)
Thermal resistance: 2.5 m²·K/W
Imperial worked example
- Layer thickness
- 3.937 in
- Thermal conductivity
- 0.04 W/(m·K)
Thermal resistance: 14.1957 h·ft²·°F/BTU
Follow the calculation
Lengths in this working are in metres.
- Thickness = 100 mm = 0.1 m.
- Layer resistance = 0.1 ÷ 0.04 = 2.5 m²·K/W.
Reviewed 19 October 2026. How calculations are checked · Report a correction