About the R-Value Calculator
R-value measures resistance to heat flow: the higher the better. Layers add up, so a wall’s total R-value is the sum of every layer plus the surface air films. U-value is simply the reciprocal, and it is the number most building codes actually specify.
Also known as: u value calculator, thermal resistance calculator, insulation r value, wall r value, r value to u value.
How to use this calculator
- Enter layer 1 thickness, layer 1 conductivity, layer 2 thickness, layer 2 conductivity 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
R = thickness ÷ conductivity · U = 1 / ΣR
Worked example
Using the values pre-filled in the calculator above:
- Layer 1 thickness = 100 mm
- Layer 1 conductivity = 0.035 W/(m·K)
- Layer 2 thickness = 100 mm
- Layer 2 conductivity = 0.7 W/(m·K)
- Layer 3 thickness = 12.5 mm
- Layer 3 conductivity = 0.25 W/(m·K)
- Surface resistances = 0.17 m²·K/W
the calculator returns:
- Total R-value: 3.22 m²·K/W
- U-value: 0.3106 W/(m²·K)
- R-value (imperial): 18.284 ft²·°F·h/Btu
- Heat loss per m² at ΔT 20 K: 6.211 W