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

5.3.2 Other unacceptable units 
There are many units besides CGS units that are outside the SI and not accepted for use with it, 
including, of course, all of the U.S. customary (that is, inch-pound) units. In the view of this Guide such 
units must strictly be avoided and SI units, their multiples or submultiples, or those units accepted or 
temporarily accepted for use with the SI (including their appropriate multiples and submultiples), must be 
used instead. This restriction also applies to the use of unaccepted special names for SI units or special 
names for multiples or submultiples of SI units, such as mho for siemens (S) and micron for micrometer 
(
μm). Table 11 gives a few examples of some of these other unacceptable units. 
 
Table 11. Examples of other unacceptable units 
Name 
Symbol 
Value in SI units 
fermi 
photometric carat 
torr 
standard atmosphere 
kilogram-force 
micron 
calorie (various) 
x unit 
stere 
gamma 
gamma (mass) 
lambda (volume) 
fermi 
metric carat 
Torr 
atm 
kgf 
μ 
cal
th
(thermochemical) 
xu 
st 
γ 
γ 
λ 
1 fermi = 1 fm = 10
−15
m 
1 metric carat = 200 mg = 2 
× 10
−4
kg 
1 Torr = (101 325/760) Pa 
1 atm = 101 325 Pa 
1 kgf = 9.806 65 N 
1 m = 1 µm = 10
−6
m 
1 cal
th
= 4.184 J 
1 xu 
≈ 0.1002 pm = 1.002 × 10
−13
m 
1 st = 1 m
3
1 γ = 1 nT = 10
−9 
T 
1 γ = 1 µg = 10
−9
kg 
1 λ = 1 µL = 10
−6
L = 10
−9
m
3
 
5.4 The terms “SI units” and “acceptable units” 
Consistent with accepted practice [1, 2], this Guide uses either the term “SI units” or “units of the 
SI” to mean the SI base units and SI coherent derived units, and multiples and submultiples of these units 
formed by using the SI prefixes. The term “acceptable units,” which is introduced in this Guide for 
convenience, is used to mean the SI units plus (a) those non-SI units accepted for use with the SI (see 
Tables 6 - 9); and (b) appropriate multiples and submultiples of such accepted non-SI units.

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