Thermal Conductivity Converter
Convert thermal conductivity between units instantly. Includes conversion tables, common equivalents, and calculation formulas.
Thermal Conductivity Converter
Calculator
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Formula: Converted Value = Input Value x (From Factor / To Factor)
Worked example โ k = 0.04 W/(m*K) = 0.277 BTU*in/(h*ft2*F), R-3.6 per inch
Formula
Converted Value = Input Value x (From Factor / To Factor)
Thermal conductivity conversion uses W/(m*K) as the base unit. Key factors: 1 BTU/(h*ft*F) = 1.730735 W/(m*K), 1 cal/(s*cm*C) = 418.68 W/(m*K). The conversion combines energy, length, and temperature unit changes. R-value (insulation rating) is inversely proportional to thermal conductivity.
Worked Examples
Example 1: Building Insulation Specification
Problem:Fiberglass insulation has k = 0.04 W/(m*K). Convert to BTU*in/(h*ft2*F) and calculate R-value per inch.
Solution:To BTU*in/(h*ft2*F): 0.04 / 0.1442279 = 0.2773 R-value per inch = 1 / (conductance per inch) R per inch = 0.0254 m / (0.04 W/(m*K)) = 0.635 m2*K/W Convert to imperial: 0.635 / 0.17611 = 3.60 ft2*F*h/BTU per inch
Result:k = 0.04 W/(m*K) = 0.277 BTU*in/(h*ft2*F), R-3.6 per inch
Example 2: Heat Sink Material Comparison
Problem:Compare copper (401 W/(m*K)) and aluminum (237 W/(m*K)) in BTU/(h*ft*F).
Solution:Copper: 401 / 1.730735 = 231.7 BTU/(h*ft*F) Aluminum: 237 / 1.730735 = 136.9 BTU/(h*ft*F) Ratio: 401/237 = 1.69, so copper conducts heat 69% better
Result:Copper: 231.7, Aluminum: 136.9 BTU/(h*ft*F)
Frequently Asked Questions
What is thermal conductivity?
Thermal conductivity (k or lambda) measures how well a material conducts heat. It is defined as the rate of heat transfer through a unit thickness of material per unit area per unit temperature difference. The SI unit is W/(m*K). High values (like copper at 401 W/(m*K)) mean the material conducts heat efficiently, while low values (like air at 0.026 W/(m*K)) indicate good insulation properties.
How do you convert W/(m*K) to BTU/(h*ft*F)?
To convert from W/(m*K) to BTU/(h*ft*F), divide the value by 1.730735. This factor combines three conversions: watts to BTU/h (1 W = 3.41214 BTU/h), meters to feet (1 m = 3.28084 ft), and Kelvin to Fahrenheit degrees (1 K = 1.8 F). The combined result gives 1 BTU/(h*ft*F) = 1.730735 W/(m*K). This conversion is common in US building insulation specifications.
What is the relationship between thermal conductivity and R-value?
R-value (thermal resistance) is the inverse of thermal conductance. For a material of thickness L and thermal conductivity k, the R-value equals L/k. Lower thermal conductivity means higher R-value (better insulation). In US customary units, R-value is in ft2*F*h/BTU. Fiberglass insulation has low k (0.04 W/(m*K)) giving high R-value, while metals have high k giving negligible R-value for the same thickness.
Why do metals have high thermal conductivity?
Metals have high thermal conductivity because their free electrons can rapidly transfer kinetic energy through the material. In metallic bonding, outer electrons are delocalized and move freely through the crystal lattice. These electrons carry thermal energy much faster than the lattice vibrations (phonons) that dominate heat transfer in non-metals. This is why copper and aluminum are preferred for heat sinks and cooking pans.
How does Fourier's law define thermal conductivity?
Fourier's law of conduction states that heat flux is proportional to the negative temperature gradient: q = -k*A*(dT/dx), where q is heat flow in watts, A is cross-sectional area, and dT/dx is the gradient through the material. Thermal conductivity k is the proportionality constant, which is why its unit W/(m*K) falls out as watts per metre of thickness per kelvin of difference. The minus sign encodes heat flowing from hot to cold.
What is the difference between thermal conductivity and thermal diffusivity?
Conductivity k describes steady-state heat flow: how much power crosses a wall once temperatures have settled. Diffusivity alpha = k/(rho*cp) describes how fast a temperature change propagates, combining conduction with the heat the material must absorb along the way. A dense material with high specific heat conducts well yet responds slowly, which is exactly the behaviour that makes masonry good thermal mass.
Where does the factor 418.68 for cal/(s*cm*degC) come from?
It is the product of two conversions. The International Table calorie is exactly 4.1868 J, giving 4.1868 W per cal/s, and the length unit changes from centimetres to metres, contributing a factor of 100 because conductivity carries length in the numerator. Multiplying 4.1868 by 100 gives 418.68 W/(m*K) per cal/(s*cm*degC). The temperature interval contributes nothing since a Celsius degree equals a kelvin.
Why does thermal conductivity change with temperature and moisture?
In non-metals heat moves by lattice vibrations, and phonon scattering shifts with temperature, so tabulated values are always tied to a stated temperature. In porous insulation the trapped air is what does the insulating, so absorbed moisture replaces air with water, whose conductivity is over twenty times higher, and performance collapses. Wet or compressed insulation therefore never delivers its rated value.
Why does real hardware perform worse than material conductivity predicts?
Two surfaces pressed together touch only at microscopic asperities, leaving air gaps that add thermal contact resistance in series with the material. In a heat sink stack that interface can dominate the whole path, which is why thermal interface materials, greases and pads exist: they displace the air with something conductive. A conductivity conversion tells you about the bulk material, not about the joints.
What separates BTU/(h*ft*F) from BTU*in/(h*ft2*F)?
Only the length unit, and the gap is a factor of 12. The first carries thickness in feet, the second in inches, so 1 BTU/(h*ft*F) equals 12 BTU*in/(h*ft2*F). That is visible in the conversion factors here: 1.730735 against 0.1442279 W/(m*K), whose ratio is exactly 12. North American insulation datasheets almost always use the inch-based form, so a number read from one convention and entered as the other is wrong by twelve times: a foam listed at 0.16 BTU*in/(h*ft2*F) is 0.16 divided by 12, about 0.013 BTU/(h*ft*F), not 0.16.
References
Reviewed for accuracy by Manoj Kumar, Mathematics Educator ยท Editorial policy
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