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

Guide for the Use of the International System of Units (SI) 
 
10 
5.2 Other Non-SI units accepted for use with the SI 
 
Because of established practice in certain fields or countries, in 1978 the CIPM considered that it 
was permissible for the following units given in Table 9, nautical mile, knot, angstrom, are, barn, bar, and 
millimeter of mercury to continue to be used with the SI. However, these units must not be introduced in 
fields where they are not presently used. Further, this Guide strongly discourages the continued use of these 
units by NIST authors except when absolutely necessary. If these units are used by NIST authors the 
values of relevant quantities shall be given in terms of SI units first followed by these non-SI units in 
parentheses.
 
The curie, roentgen, rad, and rem have been added to the NIST SP 330 [2] and Table 9 of this Guide, 
since they are in wide use in the United States, especially in regulatory documents dealing with health and 
safety. Nevertheless, this Guide strongly discourages the continued use of the curie, roentgen, rad, and rem 
and recommends that SI units should be used by NIST authors only if necessary. If these units are used by 
NIST authors the values of relevant quantities shall be given in terms of SI units first followed by these 
outdated non-SI units in parentheses. 
 
Table 9. Other non-SI units accepted for use with the SI either by the CIPM and this Guide (indicated by
*
), 
or by this Guide (indicated by
**
) 
Name 
Symbol 
Value in SI units 
nautical mile
*
knot
*
ångström
*
barn
*
bar
*
millimeter of mercury
*
curie
**
roentgen
**
rad
**
rem
**
Å
b 
bar 
mmHg 
Ci 
R 
rad
(a)
rem 
1 nautical mile = 1852 m 
1 nautical mile per hour = (1852/3600) m/s 
1 Å = 0.1 nm = 10
−10
m 
1 b = 100 fm
2
= 10
−28
m
2
1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 10
5
Pa 
1 mmHg ≈
133.322 Pa 
1 Ci = 3.7
x 10
10
Bq
1 R = 2.58 x 10
−4
C/kg 
1 rad = 1 cGy = 10
−2
Gy 
1 rem = 1 cSv = 10
−2
Sv 
(a) When there is risk of confusion with the symbol for the radian, rd may be used as the symbol for rad.