What is the exact defined value of the Boltzmann constant in J/K (as of 2019)?
Answer
1.380649 × 10^-23 J/K
As defined in the 2019 SI revision, the Boltzmann constant is exactly 1.380649 × 10^-23 J/K.
The Boltzmann constant, written k_B, connects microscopic particle energies with temperature. It appears in statistical mechanics, where it relates entropy to the number of possible microscopic arrangements through S = k_B ln W. It also links the microscopic energy scale to macroscopic quantities such as the ideal-gas constant and thermal radiation.
Before 2019, k_B was an experimentally measured quantity with an uncertainty. The revised International System of Units fixed its numerical value exactly, using it to define the kelvin alongside other defining constants. The joule and kelvin units remain part of the stated unit expression.
Values such as 1.38 × 10^-23 J/K are useful rounded forms, but they are not the exact defined value. The longer number 1.380650 is too high, while 1.3806488 reflects an older high-precision measurement rather than the post-2019 definition.