Coulomb's Constant

Coulomb's constant is 8.99×10⁹ Nm²C^-2. It is equal to 1/(4πε₀) where ε₀ is the permittivity of a vacuum which is 8.85×10^-12 Fm^-1. If the constant is to be used to calculate the Coulomb force with Coulomb's law and the electric field is not in a vacuum, then the constant will have to be recalculated with the relative permittivity instead.

References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.