Certain units are not part of the International System of Units, that is, they are outside the SI, but are important and widely used. Consistent with the recommendations of the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures), the units in this category that are accepted for use with the SI are given in Table 6.

Table 6.  Units outside the SI that are accepted for use with the SI
Name	Symbol	Value in SI units
minute (time)	min	1 min = 60 s
hour	h	1 h = 60 min = 3600 s
day	d	1 d = 24 h = 86 400 s
degree (angle)	°	1° = ( /180) rad
minute (angle)		1 = (1/60)° = (/10 800) rad
second (angle)		1 = (1/60) = (/648 000) rad
liter	L	1 L = 1 dm3 = 10-3 m3
metric ton (a)	t	1 t = 103 kg
neper	Np	1 Np = 1
bel (b)	B	1 B = (1/2) ln 10 Np (c)
electronvolt (d)	eV	1 eV = 1.602 18 x 10-19 J, approximately
unified atomic mass unit (e)	u	1 u = 1.660 54 x 10-27 kg, approximately
astronomical unit (f)	ua	1 ua = 1.495 98 x 1011 m, approximately
(a) In many countries, this unit is called "tonne.'' (b) The bel is most commonly used with the SI prefix deci: 1 dB = 0.1 B. (c) Although the neper is coherent with SI units and is accepted by the CIPM, it has not been adopted by the General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures) and is thus not an SI unit. (d) The electronvolt is the kinetic energy acquired by an electron passing through a potential difference of 1 V in vacuum. The value must be obtained by experiment, and is therefore not known exactly. (e) The unified atomic mass unit is equal to 1/12 of the mass of an unbound atom of the nuclide 12C, at rest and in its ground state. The value must be obtained by experiment, and is therefore not known exactly. (f) The astronomical unit is a unit of length. Its value is such that, when used to describe the motion of bodies in the solar system, the heliocentric gravitation constant is (0.017 202 098 95)2 ua3·d-2. The value must be obtained by experiment, and is therefore not known exactly.

Other units outside the SI that are currently accepted for use with the SI by NIST are given in Table 7. These units, which are subject to future review, should be defined in relation to the SI in every document in which they are used; their continued use is not encouraged. The CIPM currently accepts the use of all of the units given in Table 7 with the SI except for the curie, roentgen, rad, and rem. Because of the continued wide use of these units in the United States, NIST still accepts their use with the SI.