SI unit prefixes

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Even with scientific notation, very large numbers and very small numbers can be awkward to work with. That is why scientists have developed a set of standard prefixes that are attached to a unit to indicate multiples of that unit. Each of the prefixes represents a particular power of ten. There are currently 24 prefixes in use:

 

Prefixes greater than 1

Factor

Name

symbol

1030

quetta

Q

1027

ronna

R

1024

yotta

Y

1021

zetta

Z

1018

exa

E

1015

peta

P

1012

tera

T

109

giga

G

106

mega

M

103

kilo

k

102

hecto

h

101

deka

da

 

Prefixes less than 1

Factor

Name

symbol

10-1

deci

d

10-2

centi

c

10-3

milli

m

10-6

micro

μ

10-9

nano

n

10-12

pico

p

10-15

femto

f

10-18

atto

a

10-21

zepto

z

10-24

yocto

y

10-27

ronto

r

10-30

quecto

q

 

Do you have to memorize all of these? No. If you are a high school chemistry or physics student, you should know the prefixes in blue above. If you are an AP chemistry or AP physics student, you should also learn the prefixes in red. If you plan to be a scientist or an engineer, it wouldn’t hurt you to learn the rest, but the extreme prefixes are not so common.

Why do the prefixes skip three powers of 10? This allows us to use numbers that are always between 1 and 999. This is easiest to explain with an example. Let’s say we are measuring very small electrical currents. The following table shows how we would state the values without scientific notation, with scientific notation, and with prefixes. You can see how convenient this prefix method is:

Current Scientific notation Prefix notation
0.50 A 5.0 x 10-1 A 500 mA
0.050 A 5.0 x 10-2 A 50 mA
0.0050 A 5.0 x 10-3 A 5 mA
0.00050 A 5.0 x 10-4 A 500 μA
0.000050 A 5.0 x 10-5 A 50 μA
0.0000050 A 5.0 x 10-6 A 5 μA
0.00000050 A 5.0 x 10-7 A 500 nA
0.000000050 A 5.0 x 10-8 A 50 nA
0.0000000050 A 5.0 x 10-9 A 5 nA
0.00000000050 A 5.0 x 10-10 A 500 pA
Chemistry, Physics
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