where c is the defined value for the speed of light in classical vacuum in SI units, and μ 0 is the parameter that international standards organizations refer to as the magnetic constant (also called vacuum permeability or the permeability of free space). Since μ 0 has an approximate value 4π × 10 H / m, and c has the defined value 299 792 458 m⋅s, it follows that ε 0 can be expressed numerically as