The length characterizing the decrease with distance of the potential in the double layer = characteristic Debye length in the corresponding electrolyte solution = ${\kappa }^{-1}$:

$$\frac{1}{\kappa }=\sqrt{\frac{{\epsilon }_{\text{r}}{\epsilon }_{0}RT}{F^2\sum _i{c}_i{z}_i^2}}$$

(rationalized four-quantity system)

$$\frac{1}{\kappa }=\sqrt{\frac{{\epsilon }_{\text{r}}RT}{4\pi F^2\sum _i{c}_i{z}_i^2}}$$

(three-quantity electrostatic system)

where $\epsilon$ = static permittivity = ${\epsilon }_{\text{r}}{\epsilon }_0$, ${\epsilon }_{\text{r}}$ = relative static permittivity of solution; ${\epsilon }_0$ = permittivity of vacuum, $R$ = gas constant, $T$ = thermodynamic temperature, $F$ = Faraday constant, $z_{\text{i}}$ = concentration of species $i$, $z_{\text{i}}$ = ionic charge on species $i$.