Guide for the Use of the International System of Units (SI)

5.1.3 Units from International Standards 
 
There are a few highly specialized units that are given by the International Organization for 
Standardization (ISO) or the International Electrotechnical Commission (IEC) and which in the view of this 
Guide are also acceptable for use with the SI. They include the octave, phon, and sone, and units used in 
information technology, including the baud (Bd), bit (bit), erlang (E), hartley (Hart), and shannon (Sh)
3
. It 
is the position of this Guide that the only such additional units NIST authors may use with the SI are those 
given in either the International Standards on quantities and units of ISO (Ref. [4]) or of IEC (Ref. [5]).
5.1.4 Natural and atomic units 
 
In some cases, particularly in basic science, the values of quantities are expressed in terms of 
fundamental constants of nature. The two most important of these unit systems are the natural unit (n.u.) 
system used in high energy or particle physics, and the atomic unit (a.u.) system used in atomic physics and 
quantum chemistry. The use of these units with the SI is not formally accepted by the CIPM, but the CIPM 
recognizes their existence and importance. Therefore, this Guide formally accepts their use when it is 
necessary for effective communication. In such cases, the specific unit system used must be identified. 
Examples of physical quantities used as units are given in Table 8.
Table 8. Examples of physical quantities sometimes used as units 
Kind of quantity 
Physical quantity used as a unit 
Symbol 
speed 
action
mass 
electric charge 
length 
energy 
time 
speed of light in vacuum (n.u.) 
Planck constant divided by 2π (n.u.) 
electron rest mass (n.u. and a.u.) 
elementary charge (a.u.) 
Bohr radius (a.u.) 
Hartree energy (a.u.) 
ratio of action to energy (a.u.) 
c 
ħ 
m
e
 
e 
a
0 
E
h
 
ħ/ E
h
 
 
3
The symbol in parentheses following the name of the unit is its internationally accepted unit symbol, but the octave, phon, and sone 
have no such unit symbols. For additional information on the neper and bel, see Ref. [5: IEC 60027-3], and Sec. 8.7 of this Guide.

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