The Bjerrum length (after Danish chemist Niels Bjerrum 1879–1958 ) is the separation at which the electrostatic interaction between two  elementary charges is comparable in magnitude to the thermal energy scale, , where  is the Boltzmann constant and  is the absolute temperature in kelvins.  This length scale arises naturally in discussions of electrostatic, electrodynamic and electrokinetic phenomena in electrolytes, polyelectrolytes and colloidal dispersions.
In standard units, the Bjerrum length is given by
where  is the elementary charge,  is the relative dielectric constant of the medium and  is the vacuum permittivity.
For water at room temperature  , so that
In Gaussian units,  and the Bjerrum length has the simpler form
The relative permittivity ε of water decreases so strongly with temperature that the product (ε·T) decreases. Therefore, in spite of the (1/T) relation, the Bjerrum length λ increases with temperature, as shown in the graph.