# Units: SI Prefixes for Powers of Ten

Info by Collin McNeil | Last update on 2023-01-31 | Created on 2013-06-12

Within the International System of Units, some prefixes for decimal powers are defined, which I would like to list and explain in this info.

This prefixes are also called SI-prefixes, where "SI" is an abbreviation for the French term "Système international d’unités" (International System of Units).

## Table of all Prefixes

In this table, all prefixes defined so far from Quetta (Q) to Quecto (q) are listed together with their abbreviation (column "Symbol"), their power and their conversion factor:

 Prefix Symbol Power Factor Quetta Q 10^30 1.000.000.000.000.000.000.000.000.000.000 Ronna R 10^27 1.000.000.000.000.000.000.000.000.000 Yotta Y 10^24 1 000 000 000 000 000 000 000 000 Zetta Z 10^21 1 000 000 000 000 000 000 000 Exa E 10^18 1 000 000 000 000 000 000 Peta P 10^15 1 000 000 000 000 000 Tera T 10^12 1 000 000 000 000 Giga G 10^9 1 000 000 000 Mega M 10^6 1 000 000 Kilo k 10^3 1000 Hecto h 10^2 100 Deca D 10^1 10 - - 10^0 1 Deci d 10^-1 0.1 Centi c 10^-2 0.01 Milli m 10^-3 0.001 Micro µ 10^-6 0.000 001 Nano n 10^-9 0.000 000 001 Pico p 10^-12 0.000 000 000 001 Femto f 10^-15 0.000 000 000 000 001 Atto a 10^-18 0.000 000 000 000 000 001 Zepto z 10^-21 0.000 000 000 000 000 000 001 Yocto y 10^-24 0.000 000 000 000 000 000 000 001 Ronto r 10^-27 0,000.000.000.000.000.000.000.000.001 Quecto q 10^-30 0,000.000.000.000.000.000.000.000.000.001

With the exception of the units for Kilo (k) and Hecto (k), all prefixes with a conversion factor larger than 1 are written with uppercase letters, while the symbols of the prefixes with a conversion factor less than 1 are abbreviated with lowercase letters.

## Use of the Prefixes

These prefixes are used primarily for physical metric units such as meter or gram by simply prepending the prefix to the base unit (for example, decimeter or kilogram). Since numbers become cumbersome and confusing long in very large or very small scales, it is a good idea to do not write the numbers in their full length and to write an abbreviation instead.

On the one hand, this abbreviation can be made by specifying the powers that are also listed in the table (for example 3.28*10^24 meters) and on the other hand, it can be made by using these SI-prefixes (for example 3.28 yottameters). Otherwise, you have to write a number with 24 digits which is barely graspable nor readable.

But also in normal language use and in everyday life, the prefixes are often used. For instance, we can think about millimeters (0.001 meters), centimeters (0.01 meters), gigabyte (1 billion byte), kilograms (1000 grams) or millisecond (0.001 seconds).

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