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Non-SI units accepted for use with the SI, and units based on fundamental constants
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Summary
Non-SI units accepted for use with the SI, and units based on fundamental constants
Other non-SI units not recommended for use

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SI brochure, Table 7 (Section 4.1)


Table 7. Non-SI units whose values in SI units (the natural unit of speed excepted) 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



Note: For the official text, please refer to the PDF files available at:
  • http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf (in English) and
  • http://www.bipm.org/utils/common/pdf/si_brochure_8_fr.pdf (in French).
  •