where h is Planck's constant and c is the speed of light in vacuum.
Thus, the number of fundamental mechanical units (L = length, M = mass, T = time) is reduced to one: Mass is usually measured in GeV in the natural unit system. The presently adopted SI (the international system of units, also known as MKSA) is related to the natural unit system through the values of three fundamental constants:
where e is the elementary charge.
These relations allow one to calculate the conversion factors between different units.
In spite of the attempted standardization to SI, several other unit systems are in use in electromagnetism:
the Gaussian, CGS electrostatic and electromagnetic, and Heaviside-Lorentz system (for details on conversion of the units
[Jackson75]and the physical quantities ,
appendix). Electrostatic and electromagnetic units differ only by factors of c.
The parallel use of different unit systems can produce confusion, as physical quantities are defined up to multiplicative constants, which depend on the unit system. For instance, in all unit systems the force 
on a charge q in an electric field 
is 
,
hence (unit of force) = (unit of charge) 
(unit of field).
However, this does not fix the units of charge and field separately, only their product. One way to fix the unit of charge is to fix by convention the proportionality constant k in Coulomb's law
F is the force between two point charges q1 and q2
separated by a distance r.
In Gaussian units k=1, in Heaviside-Lorentz units 
,
and in SI units
with 
Fm-1
the permittivity of free space.
Strictly speaking, the SI unit C = coulomb is defined not from Coulomb's law, but from Ampère's law for the force between parallel currents, plus the relation 
between the units for charge, current and time.
The fine structure constant is
If one combines natural mechanical units, with 
,
and Gaussian electromagnetic units, then electric charge becomes dimensionless.
Thus, in these units the elementary charge is
Charge is also dimensionless in ``natural Heaviside-Lorentz'' units:
but not in natural mechanical units combined with SI electromagnetic units.
Gaussian and Heaviside-Lorentz units are different by factors of 
. Some conversion factors from SI units into Gaussian units (see [Jackson75]
for more detail) are:
Hybrid unit systems are often used in which, for example, momentum is measured in GeV/c, length in m and magnetic induction ( 
) in T = tesla. In these particular units, the elementary charge is
