Thermal Conductivity Conversion (λ)

How the calculation works

Enter a thermal conductivity and choose the unit (for example W/(m·K) or mW/(m·K)). The table shows the same value in SI, kcal/(m·h·°C), cal/(cm·s·°C), and in the US BTU/(h·ft·°F) and k-value BTU·in/(h·ft²·°F).

Formula and assumptions

Thermal conductivity λ is the heat flow through a 1 m thick layer of 1 m² at a 1 K difference. 1 BTU/(h·ft·°F) = 1,055.056 J / 3,600 s / 0.3048 m / (5/9 K) = 1.7307 W/(m·K); the k-value (per inch of thickness) = 0.14423 W/(m·K).

More worked examples

• Mineral wool λ = 0.035 W/(m·K) = 35 mW/(m·K) ≈ 0.243 BTU·in/(h·ft²·°F).
• Brick λ = 0.6 W/(m·K) ≈ 4.16 BTU·in/(h·ft²·°F).
• Concrete λ = 2.0 W/(m·K) ≈ 1.72 kcal/(m·h·°C).
• PIR k = 0.16 BTU·in/(h·ft²·°F) ≈ 0.023 W/(m·K).

λ of building materials

Common mistakes

• Mixing λ (a material property) with U (a whole layer or wall).
• Confusing BTU/(h·ft·°F) and BTU·in/(h·ft²·°F) – they differ by a factor of 12.
• Comparing λ values without the stated temperature and moisture.

Frequently asked questions

How do I convert W/(m·K) to a k-value?

Multiply by 6.9335: λ = 0.04 W/(m·K) corresponds to k ≈ 0.277 BTU·in/(h·ft²·°F).

What does mW/(m·K) mean?

A thousandth of W/(m·K): 35 mW/(m·K) = 0.035 W/(m·K). Insulation makers often quote λ in mW.

How are λ and thermal resistance R related?

R = thickness ÷ λ. A 0.10 m board with λ = 0.035 has R ≈ 2.86 m²·K/W.

Is a lower λ better?

Yes. Lower thermal conductivity means better insulation at the same thickness.

Related calculators

• Unit Converter
• U-value and insulation
• R-Value and U-Value Conversion
• Temperature Conversion