Metric Prefixes and SI Units

Contributors: JordanDee
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The Prefixes

When first learning about metric prefixes, chances are you were taught these six prefixes first:

Prefix (Symbol) Power Numeric Representation
kilo (k) 103 1,000
hecto (h) 102 100
deka (da) 101 10
no prefix 100 1 unit
deci (d) 10-1 0.1
centi (c) 10-2 0.01
milli (m) 10-3 0.001

These are what we'll consider the standard six prefixes taught in most High School science courses. You may have even learned a fun mnemonic to go along with these such as Kangaroos Have Dirty Underwear During Cold Months. However, as you'll soon see, when learning about electronics and computer science, the range of prefixes well exceeds the standard six. While these prefixes cover a rang of 10-3 to 103, many electronic values can have a much larger range.

Describing the Large

Prefix (Symbol) Power Numeric Representation
yotta (Y) 1024 1 septillion
zetta (Z) 1021 1 sextillion
exa (E) 1018 1 quintillion
peta (P) 1015 1 quadrillion
tera (T) 1012 1 trillion
giga (G) 109 1 billion
mega (M) 106 1 million
kilo (k) 103 1 thousand
no prefix 100 1 unit

These above prefixes dramatically help describe quanities of units in large amounts. Instead of saying 3,200,000,000 Hertz, you can say 3.2 GigaHertz, or 3.2 GHz for shorthand written notation. This allows us to describe incredibly large numbers of units succinctly. There are also prefixes for helping communicate tiny numbers as well.

Describing the Small

Prefix (Symbol) Power Numeric Representation
no prefix 100 1 unit
milli (m) 10-3 1 thousandth
micro (µ) 10-6 1 millionth
nano (n) 10-9 1 billionth
pico (p) 10-12 1 trillionth
femto (f) 10-15 1 quadrillionth
atto (a) 10-18 1 quintillionth
zepto (z) 10-21 1 sextillionth
yocto (y) 10-24 1 septillionth

Now, instead one trillionth of a second, it can be referred to as a picosecond. One thing to notice about the prefixes for small values, is that their shorthand notations are all lower case while the large number prefixes are upper case (with the exception of kilo-*, hecto- and deca-). This allows you to distinguish between the two when they use the same letter. As an example, one mW (milliwatt) does not equal one MW (megawatt).

*Note: Since the upper case 'K' was already used to describe Kelvins, a lower case 'k' was chosen to represent the prefix kilo-. As you'll see in the Bits and Bytes section, there is also some confusion with k and K when dealing with the binary (base 2) prefixes.