8.854187187... x 10^-12

The physical constant ε0, commonly called the electric constant, relates the units for electric charge to mechanical quantities such as length and force.
The value is defined as 1/(c0^2 x μ0), where c0 is the speed of light in vacuum and μ0 is the magnetic constant.

The most common use of this constant is Gauss's law; Φ = Q/ε0.