SI prefixes
See also prefixes for binary multiples adopted by the IEC.
The 20 SI prefixes used to form decimal multiples and submultiples of SI units
are given in Table 5.
Table 5. SI prefixes
Factor Name
1024 yotta
1021 zetta
1018 exa
1015 peta
1012 tera
109
giga
6
10
mega
3
10
kilo
2
10
hecto
1
10
deka
Symbol
Y
Z
E
P
T
G
M
k
h
da
Factor Name
10-1
deci
-2
10
centi
-3
10
milli
-6
10
micro
-9
10
nano
-12
10
pico
-15
10
femto
-18
10
atto
-21
10
zepto
-24
10
yocto
Symbol
d
c
m
µ
n
p
f
a
z
y
It is important to note that the kilogram is the only SI unit with a prefix as part
of its name and symbol. Because multiple prefixes may not be used, in the
case of the kilogram the prefix names of Table 5 are used with the unit name
"gram" and the prefix symbols are used with the unit symbol "g." With this
exception, any SI prefix may be used with any SI unit, including the degree
Celsius and its symbol °C.
Example 1: 10-6 kg = 1 mg (one milligram), but not 10-6 kg = 1 µkg (one microkilogram)
Example 2: Consider the earlier example of the height of the Washington Monument.
We may write hW = 169 000 mm = 16 900 cm = 169 m = 0.169 km using the
millimeter (SI prefix milli, symbol m), centimeter (SI prefix centi, symbol c),
or kilometer (SI prefix kilo, symbol k).
Because the SI prefixes strictly represent powers of 10, they should not be
used to represent powers of 2. Thus, one kilobit, or 1 kbit, is 1000 bit
and not210 bit = 1024 bit. To alleviate this ambiguity, prefixes for binary
multiples have been adopted by the International Electrotechnical
Commission (IEC) for use in information technology.