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

Table 5. SI prefixes 
Factor Prefix 
Name Symbol 
Factor Prefix 
Name 
Symbol 
10
24
= (10
3
)
8 
10
21
= (10
3
)
7 
10
18
= (10
3
)
6 
10
15
= (10
3
)
5 
10
12
= (10
3
)
4 
10
9
= (10
3
)
3 
10
6
= (10
3
)
2 
10
3
= (10
3
)
1 
10
2 
10
1 
yotta 
zetta 
exa 
peta 
tera 
giga 
mega 
kilo 
hecto 
deka 
Y 
Z 
E 
P 
T 
G 
M 
k 
h 
da 
10
−1
10
−2
10
−3
= (10
3
)
−1 
10
−6
= (10
3
)
−2 
10
−9
= (10
3
)
−3 
10
−12
= (10
3
)
−4 
10
−15
= (10
3
)
−5 
10
−18
= (10
3
)
−6 
10
−21
= (10
3
)
−7 
10
−24
= (10
3
)
−8
deci 
centi 
milli 
micro 
nano 
pico 
femto 
atto 
zepto 
yocto 
d 
c 
m 
µ 
n 
p 
f 
a 
z 
y 
Note: Alternative definitions of the SI prefixes and their symbols are not permitted. For example, it is unacceptable to use kilo (k) to 
represent 2
10
= 1024, mega (M) to represent 2
20
= 1 048 576, or giga (G) to represent 2
30
= 1 073 741 824. See the note to 
Ref. [5] on page 74 for the prefixes for binary powers adopted by the IEC. 
5
Units Outside the SI 
Units that are outside the SI, that is, non-SI units, may be divided into three categories: 
— those units that are accepted for use with the SI by the CIPM and hence this Guide; 
— those units that are not accepted for use with the SI by the CIPM, but are temporarily accepted 
for use with the SI by this Guide; and 
— those units that are not accepted for use with the SI by either the CIPM or this Guide and in the 
view of this Guide must strictly be avoided. 
5.1 Units accepted for use with the SI 
The following four sections discuss in detail the units this Guide accepts for use with the SI. 

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Guide for the Use of the International System of Units (SI) 
5.1.1 Hour, degree, liter, and the like 
 
Certain units that are not part of the SI are essential and used so widely that they are accepted by the 
CIPM, and thus by this Guide, for use with the SI [2, 3]. These units are given in Table 6. The combination 
of units of this table with SI units to form derived units should be restricted to special cases in order not to 
lose the advantages of the coherence of SI units. (The use of SI prefixes with the units of Table 6 is 
discussed in Sec. 6.2.8.) 
Additionally, 
this 
Guide recognizes that situations on occasion will require the use of time-related 
units other than those given in Table 6; such as using intervals of time be expressed in weeks, months, or 
years. In such cases, if a standardized symbol for the unit is not available, the name of the unit should be 
written out in full. (See Sec. 8.1 for a suggestion regarding the symbol for year and Chapter 9 for the rules 
and style conventions for spelling unit names.)