Units & conversions

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Units

Dimensions for thermal conductivity are W/m.K.

In the temperature range 100-3000 K conductivity can vary from 10-4 to 107 W/ m.K.

Dimensions for thermal diffusivity are m2/s.

Its range is not as great as for conductivity.

Diffusivity depends on thermal conductivity, specific heat capacity and density.

Unit conversions

Thermal conductivity involves heat flow per unit area (W/m2), length or thickness (m) and temperature difference (K). The SI unit is W / (m.K), that is 'watts per metre kelvin'. Historically, due to common usage of the Imperial and Metric Systems in different countries, and by different scientific disciplines and industries, a variety of other units are still in use for the property. So there can be confusion when examining data. For gases, fluids and thermal insulation (especially in vacuum) that have very low thermal conductivities, a more practical unit is mW / (m.K), which avoids use of too many zeroes.

Thermal diffusivity is a combination of units involving thermal conductivity divided by the specific heat capacity and density, yielding a value in units of m2 / s. However, it is often quoted in values of cm2 / s or mm2 / s, for very low values, again to avoid too many zeroes. The table below contains appropriate conversion factors for the units in common use.

 Conversion table for common imperial and metric units MULTIPLY by appropriate factor to OBTAIN W/(m·K) W/(cm·K) cal/(cm·s·K) kcal/(m·h·K) Btu/(ft·h·F) Btu·in/(h·ft2·F) 1W/(m·°C) 1 0.01 0.002388 0.8598 0.5778 6.933 1W/(cm·°C) 100 1 0.2388 85.98 57.78 693.3 1cal/(cm·s·°C) 418.6 4.186 1 360 241.9 2903 1kcal/(m·h·°C) 1.163 0.0163 0.002778 1 0.672 8.064 1Btu/(ft·h·F) 1.731 0.01731 0.04134 1.488 1 12 1Btu·in/(ft2·h·F) 0.1442 0.001442 0.0003445 0.124 0.08333 1

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