giga : Computer Hardware Buyers’ Glossary

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giga
There are a series of Greek, Latin, Danish and Spanish prefixes used for specifying large numbers. The prefixes have different meaning when talking about RAM (Random Access Memory) than when talking about disk space.

Two Kinds of Gigabytes

Hard disk capacities are measured in decimal gigabytes, in billions of bytes, namely 109 = 1000x1000x1000 = 1,000,000,000 bytes.
106 means 1 followed by 6 zeros, i.e. 1,000,000.
10-6 means 1 in the 6th decimal place, i.e. .000001.

However, Windows measures hard disk capacity using the binary RAM definition.

For measuring RAM, a gigabyte is a little over a billion bytes, or more precisely, 1024 megabytes = 230 bytes = 1024x1024x1024 bytes = 1,073,741,824 bytes. A byte is approximately one character of information.

Two Kinds of Megabytes

A megabyte on disk is precisely a million, namely 109 =1000x1000x1000 = 1,000,000 bytes. For RAM it is a little over a million bytes, namely 220 = 1024 × 1024 = 1,048,576 bytes.

SI (Systèm Internationale (metric)) Prefixes

SI prefixes
Factor Name  Symbol
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
  
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
binary prefixes
lakh
NIST
Proposed extended prefixes
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