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SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]

Chapter 4: Units outside the SI

    The International System of Units, the SI, is a system of units, adopted by the CGPM, which provides the internationally agreed reference in terms of which all other units are now defined. It is recommended for use throughout science, technology, engineering, and commerce. The SI base units, and the SI coherent derived units, including those with special names, have the important advantage of forming a coherent set, with the effect that unit conversions are not required when inserting particular values for quantities into quantity equations. Because the SI is the only system of units that is globally recognized, it also has a clear advantage for establishing a worldwide dialogue. Finally, it simplifies the teaching of science and technology to the next generation if everyone uses this system.

    Nonetheless it is recognized that some non-SI units still appear in the scientific, technical and commercial literature, and will continue to be used for many years. Some non-SI units are of historical importance in the established literature. Other non-SI units, such as the units of time and angle, are so deeply embedded in the history and culture of the human race that they will continue to be used for the foreseeable future. Individual scientists should also have the freedom to sometimes use non-SI units for which they see a particular scientific advantage in their work. An example of this is the use of CGS-Gaussian units in electromagnetic theory applied to quantum electrodynamics and relativity. For these reasons it is helpful to list some of the more important non-SI units, as is done below. However, if these units are used it should be understood that the advantages of the SI are lost.

    The inclusion of non-SI units in this text does not imply that the use of non-SI units is to be encouraged. For the reasons already stated SI units are generally to be preferred. It is also desirable to avoid combining non-SI units with units of the SI; in particular, the combination of non-SI units with the SI to form compound units should be restricted to special cases in order not to compromise the advantages of the SI. Finally, when any of the non-SI units in Tables 7, 8, and 9 are used, it is good practice to define the non-SI unit in terms of the corresponding SI unit.

Section 4.1: Non-SI units accepted for use with the SI, and units based on fundamental constants

The CIPM (2004) has revised the classification of non-SI units from that in the previous (7th) edition of this Brochure. Table 6 gives non-SI units that are accepted for use with the International System by the CIPM, because they are widely used with the SI in matters of everyday life. Their use is expected to continue indefinitely, and each has an exact definition in terms of an SI unit. Tables 7, 8 and 9 contain units that are used only in special circumstances. The units in Table 7 are related to fundamental constants, and their values have to be determined experimentally. Tables 8 and 9 contain units that have exactly defined values in terms of SI units, and are used in particular circumstances to satisfy the needs of commercial, legal, or specialized scientific interests. It is likely that these units will continue to be used for many years. Many of these units are also important for the interpretation of older scientific texts. Each of the Tables 6, 7, 8 and 9 is discussed in turn below.

Table 6: Non-SI units accepted for use with the International System of Units
Table 7: Non-SI units whose values in SI units must be obtained experimentally

Tables 8 and 9 contain non-SI units that are used by special interest groups for a variety of different reasons. Although the use of SI units is to be preferred for reasons already emphasized, authors who see a particular advantage in using these non-SI units should have the freedom to use the units that they consider to be best suited to their purpose. Since, however, SI units are the international meeting ground in terms of which all other units are defined, those who use units from Tables 8 and 9 should always give the definition of the units they use in terms of SI units.

Table 8: Other non-SI units
Table 9. Non-SI units associated with the CGS and the CGS-Gaussian system of units

    Table 6: Non-SI units accepted for use with the International System of Units

Table 6 includes the traditional units of time and angle. It also contains the hectare, the litre, and the tonne, which are all in common everyday use throughout the world, and which differ from the corresponding coherent SI unit by an integer power of ten. The SI prefixes are used with several of these units, but not with the units of time.

Table 6. Non-SI units accepted for use with the International System of Units

Quantity Name of unit Symbol for unit Value in SI units
time minute min 1 min = 60 s
hour (a) h 1 h = 60 min = 3600 s
day d 1 d = 24 h = 86 400 s
plane angle degree (b,c) ° 1° = (pi/180) rad
minute ' 1' = (1/60)° = (pi/10 800) rad
second (d) '' 1'' = (1/60)' = (pi/648 000) rad
area hectare (e) ha 1 ha = 1 hm2 = 104 m2
volume litre (f) L, l 1 L = 1 l = 1 dm3 = 103 cm3 = 10–3 m3
mass tonne (g) t 1 t = 103 kg
length astronomical unit (h) au 1 au = 149 597 870 700 m

(a) The symbol of this unit is included in Resolution 7 of the 9th CGPM (1948).
(b) ISO 80000-3:2006 recommends that the degree be divided decimally rather than using the minute and the second. For navigation and surveying, however, the minute has the advantage that one minute of latitude on the surface of the Earth corresponds (approximately) to one nautical mile (defined in Table 8).
(c) The gon (or grad, where grad is an alternative name for the gon) is an alternative unit of plane angle to the degree, defined as (pi/200) rad. Thus there are 100 gon in a right angle. The potential value of the gon in navigation is that because the distance from the pole to the equator of the Earth is approximately 10 000 km, 1 km on the surface of the Earth subtends an angle of one centigon at the centre of the Earth. However the gon is rarely used.
(d) For applications in astronomy, small angles are measured in arcseconds (i.e. seconds of plane angle), denoted by the symbol as or ''; also used are milliarcseconds, microarcseconds, and picoarcseconds, denoted by the symbols mas, µas, and pas, respectively, where arcsecond is an alternative name for second of plane angle.
(e) The unit hectare, and its symbol ha, were adopted by the CIPM in 1879 (PV, 1879, 41). The hectare is used to express land area.
(f) The litre, and the symbol lower-case l, were adopted by the CIPM in 1879 (PV, 1879, 41). The alternative symbol, capital L, was adopted by the 16th CGPM (1979, Resolution 6) in order to avoid the risk of confusion between the letter l (el) and the numeral 1 (one).
(g) The tonne, and its symbol t, were adopted by the CIPM in 1879 (PV, 1879, 41). In English speaking countries this unit is usually called "metric ton".
(h) The astronomical unit of length was redefined by the XXVIII General Assembly of the International Astronomical Union (Resolution B2, 2012).

[ updated 2014 ]
 

    Table 7: Non-SI units whose values in SI units must be obtained experimentally

Table 7 contains units whose values in SI units have to be determined experimentally, and thus have an associated uncertainty. Except for the astronomical unit, all other units in Table 7 are related to fundamental physical constants. The first three units, the non-SI units electronvolt, symbol eV, dalton or unified atomic mass unit, symbol Da or u, respectively, and the astronomical unit, symbol ua, have been accepted for use with the SI by the CIPM. The units in Table 7 play important roles in a number of specialized fields in which the results of measurements or calculations are most conveniently and usefully expressed in these units. For the electronvolt and the dalton the values depend on the elementary charge e and the Avogadro constant NA, respectively.

There are many other units of this kind, because there are many fields in which it is most convenient to express the results of experimental observations or of theoretical calculations in terms of fundamental constants of nature. The two most important of such unit systems based on fundamental constants 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. In the n.u. system, the base quantities for mechanics are speed, action, and mass, for which the base units are the speed of light in vacuum c0, the Planck constant h divided by 2pi, called the reduced Planck constant with symbol , and the mass of the electron me, respectively. In general these units are not given any special names or symbols but are simply called the n.u. of speed, symbol c0, the n.u. of action, symbol , and the n.u. of mass, symbol me. In this system, time is a derived quantity and the n.u. of time is a derived unit equal to the combination of base units /mec02. Similarly, in the a.u. system, any four of the five quantities charge, mass, action, length, and energy are taken as base quantities. The corresponding base units are the elementary charge e, electron mass me, action , Bohr radius (or bohr) a0, and Hartree energy (or hartree) Eh, respectively. In this system, time is again a derived quantity and the a.u. of time a derived unit, equal to the combination of units /Eh. Note that a0 = alpha/(4piRinfinity), where alpha is the fine-structure constant and Rinfinity is the Rydberg constant; and Eh = e2/(4piepsilon0a0) = 2Rhc0 = alpha2mec02, where epsilon0 is the electric constant and has an exact value in the SI.

For information, these ten natural and atomic units and their values in SI units are also listed in Table 7. Because the quantity systems on which these units are based differ so fundamentally from that on which the SI is based, they are not generally used with the SI, and the CIPM has not formally accepted them for use with the International System. To ensure understanding, the final result of a measurement or calculation expressed in natural or atomic units should also always be expressed in the corresponding SI unit. Natural units (n.u.) and atomic units (a.u.) are used only in their own special fields of particle and atomic physics, and quantum chemistry, respectively. Standard uncertainties in the least significant digits are shown in parenthesis after each numerical value.

Table 7. Non-SI units whose values in SI units must be obtained experimentally

Quantity Name of unit Symbol for unit Value in SI units (a)
Units accepted for use with the SI
energy electronvolt (b) eV 1 eV = 1.602 176 565(35) x 10–19 J
mass dalton, (c) Da 1 Da = 1.660 538 921(73) x 10–27 kg
unified atomic mass unit u 1 u = 1 Da
Natural units (n.u.)
speed n.u. of speed
(speed of light in vacuum)
c0 299 792 458 m/s (exact)
action n.u. of action
(reduced Planck constant)
h bar 1.054 571 726(47) x 10–34 J s
mass n.u. of mass
(electron mass)
me 9.109 382 91(40) x 10–31 kg
time n.u. of time h bar/(mec02) 1.288 088 668 33(83) x 10–21 s
Atomic units (a.u.)
charge a.u. of charge
(elementary charge)
e 1.602 176 565(35) x 10–19 C
mass a.u. of mass
(electron mass)
me 9.109 382 91(40) x 10–31 kg
action a.u. of action
(reduced Planck constant)
h bar 1.054 571 726(47) x 10–34 J s
length a.u. of length, bohr
(Bohr radius)
a0 0.529 177 210 92(17) x 10–10 m
energy a.u. of energy, hartree
(Hartree energy)
Eh 4.359 744 34(19) x 10–18 J
time a.u. of time h bar/Eh 2.418 884 326 502(12) x 10–17 s

[ updated 2014 ]

    Table 8: Other non-SI units

Table 8 also gives the units of logarithmic ratio quantities, the neper, bel, and decibel. These are dimensionless units that are somewhat different in their nature from other dimensionless units, and some scientists consider that they should not even be called units. They are used to convey information on the nature of the logarithmic ratio quantity concerned. The neper, Np, is used to express the values of quantities whose numerical values are based on the use of the neperian (or natural) logarithm, ln = loge. The bel and the decibel, B and dB, where 1 dB = (1/10) B, are used to express the values of logarithmic ratio quantities whose numerical values are based on the decadic logarithm, lg = log10. The way in which these units are interpreted is described in footnotes (g) and (h) of Table 8. The numerical values of these units are rarely required. The units neper, bel, and decibel have been accepted by the CIPM for use with the International System, but are not considered as SI units.

The SI prefixes are used with two of the units in Table 8, namely, with the bar (e.g. millibar, mbar), and with the bel, specifically for the decibel, dB. The decibel is listed explicitly in the table because the bel is rarely used without the prefix.

Table 8. Other non-SI units

Quantity Name of unit Symbol for unit Value in SI units
pressure bar (a) bar 1 bar = 0.1 MPa = 100 kPa = 105 Pa
millimetre of mercury (b) mmHg 1 mmHg approximately equal to 133.322 Pa
length ångström (c) Å 1 Å = 0.1 nm = 100 pm = 10–10 m
distance nautical mile (d) M 1 M = 1852 m
area barn (e) b 1 b = 100 fm2 = (10–12 cm)2 = 10–28 m2
speed knot (f) kn 1 kn = (1852/3600) m/s
logarithmic ratio quantities neper (g,i) Np [see footnote (j) regarding the numerical value of the neper, the bel and the decibel]
bel (h,i) B
decibel (h,i) dB

(a) The bar and its symbol are included in Resolution 7 of the 9th CGPM (1948). Since 1982 one bar has been used as the standard pressure for tabulating all thermodynamic data. Prior to 1982 the standard pressure used to be the standard atmosphere, equal to 1.013 25 bar, or 101 325 Pa.
(b) The millimetre of mercury is a legal unit for the measurement of blood pressure in some countries.
(c) The ångström is widely used by x-ray crystallographers and structural chemists because all chemical bonds lie in the range 1 to 3 ångströms. However it has no official sanction from the CIPM or the CGPM.
(d) The nautical mile is a special unit employed for marine and aerial navigation to express distance. The conventional value given here was adopted by the First International Extraordinary Hydrographic Conference, Monaco 1929, under the name "International nautical mile". As yet there is no internationally agreed symbol, but the symbols M, NM, Nm, and nmi are all used; in the table the symbol M is used. The unit was originally chosen, and continues to be used, because one nautical mile on the surface of the Earth subtends approximately one minute of angle at the centre of the Earth, which is convenient when latitude and longitude are measured in degrees and minutes of angle.
(e) The barn is a unit of area employed to express cross sections in nuclear physics.
(f) The knot is defined as one nautical mile per hour. There is no internationally agreed symbol, but the symbol kn is commonly used.
(g) The statement LA = n Np (where n is a number) is interpreted to mean that ln(A2/A1) = n. Thus when LA = 1 Np, A2/A1 = e. The symbol A is used here to denote the amplitude of a sinusoidal signal, and LA is then called the neperian logarithmic amplitude ratio, or the neperian amplitude level difference.
(h) The statement LX = m dB = (m/10) B (where m is a number) is interpreted to mean that lg(X/X0) = m/10. Thus when LX = 1 B, X/X0 = 10, and when LX = 1 dB, X/X0 = 101/10. If X denotes a mean square signal or power-like quantity, LX is called a power level referred to X0.
(i) In using these units it is important that the nature of the quantity be specified, and that any reference value used be specified. These units are not SI units, but they have been accepted by the CIPM for use with the SI.
(j) The numerical values of the neper, bel, and decibel (and hence the relation of the bel and the decibel to the neper) are rarely required. They depend on the way in which the logarithmic quantities are defined.
 

    Table 9: Non-SI units associated with the CGS and the CGS-Gaussian system of units

Table 9 differs from Table 8 only in that the units in Table 9 are related to the older CGS (centimetre-gram-second) system of units, including the CGS electrical units. In the field of mechanics, the CGS system of units was built upon three quantities and their corresponding base units: the centimetre, the gram, and the second. The CGS electrical units were still derived from only these same three base units, using defining equations different from those used for the SI. Because this can be done in different ways, it led to the establishment of several different systems, namely the CGS-ESU (electrostatic), the CGS-EMU (electromagnetic), and the CGS-Gaussian unit systems. It has always been recognized that the CGS-Gaussian system, in particular, has advantages in certain areas of physics, particularly in classical and relativistic electrodynamics (9th CGPM, 1948, Resolution 6). Table 9 gives the relations between these CGS units and the SI, and lists those CGS units that were assigned special names. As for the units in Table 8, the SI prefixes are used with several of these units (e.g. millidyne, mdyn; milligauss, mG, etc.).

Table 9. Non-SI units associated with the CGS and the CGS-Gaussian system of units

Quantity Name of unit Symbol for unit Value in SI units
energy erg (a) erg 1 erg = 10–7 J
force dyne (a) dyn 1 dyn = 10–5 N
dynamic viscosity poise (a) P 1 P = 1 dyn s cm–2 = 0.1 Pa s
kinematic viscosity stokes St 1 St = 1 cm2 s–1 = 10–4 m2 s–1
luminance stilb (a) sb 1 sb = 1 cd cm–2 = 104 cd m–2
illuminance phot ph 1 ph = 1 cd sr cm–2 = 104 lx
acceleration gal (b) Gal 1 Gal = 1 cm s–2 = 10–2 m s–2
magnetic flux maxwell (c) Mx 1 Mx = 1 G cm2 = 10–8 Wb
magnetic flux density gauss (c) G 1 G = 1 Mx cm–2 = 10–4 T
magnetic field œrsted (c) Oe 1 Oe corresponds to (103/4pi) A m–1

(a) This unit and its symbol were included in Resolution 7 of the 9th CGPM (1948).
(b) The gal is a special unit of acceleration employed in geodesy and geophysics to express acceleration due to gravity.
(c) These units are part of the so-called "electromagnetic" three-dimensional CGS system based on unrationalized quantity equations, and must be compared with care to the corresponding unit of the International System which is based on rationalized equations involving four dimensions and four quantities for electromagnetic theory. The magnetic flux, phi, and the magnetic flux density, B, are defined by similar equations in the CGS system and the SI, so that the corresponding units can be related as in the table. However, the unrationalized magnetic field, H (unrationalized) = 4pi x H (rationalized). The equivalence symbol corresponds to is used to indicate that when H (unrationalized) = 1 Oe, H (rationalized) = (103/4pi) A m–1.
 

Section 4.2: Other non-SI units not recommended for use

There are many more non-SI units, which are too numerous to list here, which are either of historical interest, or are still used but only in specialized fields (for example, the barrel of oil) or in particular countries (the inch, foot, and yard). The CIPM can see no case for continuing to use these units in modern scientific and technical work. However, it is clearly a matter of importance to be able to recall the relation of these units to the corresponding SI units, and this will continue to be true for many years. The CIPM has therefore decided to compile a list of the conversion factors to the SI for such units and to make this available on the BIPM website at

www.bipm.org/en/si/si_brochure/chapter4/conversion_factors.html.