The impedance of free space (more correctly, the wave-impedance of a plane wavein free space) equals the product of the vacuum permeability or magnetic constant μ0 and the speed of light in vacuumc0. Since the numerical values of the magnetic constant and of the speed of light are fixed by the definitions of the ampere and the metre respectively, the exact value of the impedance of free space is likewise fixed by definition relative to the S.I. base units.
The reciprocal of is sometimes referred to as the admittance of free space, and represented by the symbol .
Since 1948, the SI unit ampere has been defined by choosing the numerical value of μ0 to be exactly 4π×10−7H/m. Similarly, since 1983 the SI metre has been defined by choosing the value of c0 to be 299 792 458 m/s. Consequently
It is very common in textbooks and learned papers written before about 1990 to substitute the approximate value for . This is equivalent to taking the speed of light to be 3×108 m/s. For example, Cheng 1989 states that the radiation resistance of a Hertzian dipole is
This practice may be recognized from the resulting discrepancy in the units of the given formula. Consideration of the units, or more formally dimensional analysis, may be used to restore the formula to a more exact form—in this case to