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/(mK)

W/(cmK)

cal/(cmsK)

kcal/(mhK)

 Btu/(fthF)

Btuin/(hft2F)

1W/(mC)

1

0.01

0.002388

0.8598

0.5778

6.933

1W/(cmC)

100

1

0.2388

85.98

57.78

693.3

1cal/(cmsC)

418.6

4.186

1

360

241.9

2903

1kcal/(mhC)

1.163

0.0163

0.002778

1

0.672

8.064

1Btu/(fthF)

1.731

0.01731

0.04134

1.488

1

12

1Btuin/(ft2hF)

0.1442

0.001442

0.0003445

0.124

0.08333

1



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