The Handy Math Answer Book

American system metric prefix/symbol number

1 trillion tera- / T- 1012

1 billion giga- / G- 109

1 million mega- / M- 106

1 thousand kilo- / k- 103

1 hundred hecto- / h- 102

1 ten deka- / da- 10

1 tenth deci- / d- 10 -1

1 hundredth centi- / c- 10 ^2

1 thousandth milli- / m- 10 ^3

1 millionth micro-/  10 ^6

1 billionth nano- / n- 10 ^9

1 trillionth pico- / p- 10 ^12

1 quadrillionth femto- / f- 10 ^15

1 quintillionth atto- / a- 10 ^18

1 sextillionth zepto- / z- 10 ^21

1 septillionth yocto- / y- 10 ^24

It is interesting to note that “deca-” is the recommended spelling by the Interna-

tional System (SI), but the United States National Institute of Standards and Technol-

ogy spells the prefix “deka-.” Thus, either one is considered by most references to be

correct. There are also spelling variations between countries; for example, in Italy,

hecto- is spelled etto- and kilo- is spelled chilo-. But the symbols remain standard

through all languages. As for other numbers in the metric system—such as 10^5 or

10 ^5 )—there are no set names or prefixes.

Why are some prefix names differentin measurements?
The main reason why a prefix name would differ has to do with pronunciation and
vowels: If the first letter of the unit name is a vowel and the pronunciation is difficult,
the last letter of the prefix is omitted. For example, a metric measurement of 100 ares
52 (2.471 acres) is a hectare (not hectoare) and 1 million ohms is a megohm (not

Why is the word “centimillion” incorrect?

C

entimillion is a word sometimes incorrectly used to mean 100 million (10^8 ).

But the metric prefix “centi-” means 1/100, not 100. There are ways to name

this number: 100 million could be called a hectomillion; in the United States, it

could be called a decibillion.