The constant of proportionality, 1 4 π π ε ε 0 , in Coulomb's law: F 1 = q 1 q 2 4 π π ε ε 0 r ^ ^ 12 | r 12 | 2 is a consequence of historical choices for units. The constant ε ε 0 is the vacuum electric permittivity. Using the CODATA 2018 recommended value for ε ε 0 , the Coulomb constant is k e = 1 4 π π ε ε 0 = 8.987 551 792 3 ( 14 ) × × 10 9 N ⋅ ⋅ m 2 ⋅ ⋅ C − − 2 .