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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 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 53 (14) x 10–19 J
mass dalton, (c) Da 1 Da = 1.660 538 86 (28) x 10–27 kg
unified atomic mass unit u 1 u = 1 Da
length astronomical unit (d) ua 1 ua = 1.495 978 706 91 (6) x 1011 m
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 68 (18) x 10–34 J s
mass n.u. of mass
(electron mass)
me 9.109 3826 (16) x 10–31 kg
time n.u. of time h bar/(mec02) 1.288 088 6677 (86) x 10–21 s
Atomic units (a.u.)
charge a.u. of charge
(elementary charge)
e 1.602 176 53 (14) x 10–19 C
mass a.u. of mass
(electron mass)
me 9.109 3826 (16) x 10–31 kg
action a.u. of action
(reduced Planck constant)
h bar 1.054 571 68 (18) x 10–34 J s
length a.u. of length, bohr
(Bohr radius)
a0 0.529 177 2108 (18) x 10–10 m
energy a.u. of energy, hartree
(Hartree energy)
Eh 4.359 744 17 (75) x 10–18 J
time a.u. of time h bar/Eh 2.418 884 326 505 (16) 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).
  •