†The value is exact but not expressible as a finite decimal; approximated to 9 decimal places only.
Since  is a proportionality factor between temperature and energy, its numerical value depends on the choice of units for energy and temperature.  The small numerical value of the Boltzmann constant in SI units means a change in temperature by 1 K only changes a particle's energy by a small amount. A change of  is defined to be the same as a change of .  The characteristic energy  is a term encountered in many physical relationships.
The Boltzmann constant sets up a relationship between wavelength and temperature (dividing hc/k by a wavelength gives a temperature) with one micrometer being related to , and also a relationship between voltage and temperature (kT in units of eV corresponds to a voltage) with one volt being related to . The ratio of these two temperatures,  /  ≈ 1.239842, is the numerical value of hc in units of eV⋅μm.