From Mac Guides
There are a number of units used for various computing related measurements. This article summarizes the most common.
There is much confusion over the definitions of the terms kilobyte, megabyte, gigabyte etc. The recommended definition by international standards organizations such as the IEC, IEEE and ISO is that 1 kilobyte = 1000 bytes (10^3). This is similar to the use of the "kilo" prefix for other units, such as kilogram (1000 grams). When a context uses this definition, the term used for 1024 bytes is a kibibyte (unit KiB), with larger sizes being a mebibyte (MiB), gibibyte (GiB), tebibyte (TiB) and so on, where each unit is 1024 (2^10) times as large as the preceding one.
However, these definitions are rarely used in the mainstream. In general, a kilobyte will refer to the 1024 bytes (a kibibyte), while a megabyte is the same as a mebibyte, and so on. This definition is used by most software, such as by Finder when displaying file sizes, and it is also used for RAM memory modules. The main exceptions are in marketing, where most hardware is advertised using the less common, standards-based definitions, and when using bits, which also uses these definitions in most cases. More information about this can be found in the Bits And Bytes article.
This article will use the mainstream definition of these units. This only affects the Space and Memory and Transfer Rates and Bitrates sections, the Processor Speeds and Frequencies and FLOPS sections are unaffected.
Space and Memory
|Bit||b||The smallest amount of space, which contains either a 1 or a 0.||Used for storing boolean values (ie if something is on or off), eg whether a checkbox is checked.|
|Byte||B||Equivalent to 8 bits (see Bits And Bytes).||Can store integers from 0 to 255 or ASCII characters.|
|Kilobyte||KB||Equivalent to 1024 bytes.||Can store a small plain text file or a very small, compressed image.|
|Megabyte||MB||Equivalent to 1024 KB or 1048576 bytes.||Can store over a million characters. A short (about 1 minute) MP3 file or very short, compressed video clip might be this size.|
|Gigabyte||GB||Equivalent to 1024 MB or over a billion bytes.||Can store about 250 MP3 files, or a compressed movie length video. Used as a unit for modern hard disks. Hard disks were about this size in the mid to late 1990's.|
|Terabyte||TB||Equivalent to 1024 GB or over a trillion bytes.||A terabyte is a huge amount of data, larger than all hard disks commonly available. Hard drives could realistically reach this capacity in a few years.|
|Petabyte||PB||Equivalent to 1024 TB or over a quadrillion bytes.||A petabyte of storage is only needed for projects of extremely large scale. Google and the Internet Archive Wayback Machine reportedly both store over a petabyte of data.|
|Exabyte||EB||Equivalent to 1024 PB or over a quintillion bytes.||The world's total printed material could be stored in about 5 exabytes (source).|
|Zettabyte||ZB||Equivalent to 1024 EB or over a sextillion bytes.||A zettabyte is so large that it currently has no practical use.|
|Yottabyte||YB||Equivalent to 1024 ZB, or over a septillion bytes. Equivalent to about a trillion 120 GB hard disks.||A yottabyte could store almost any imaginable digital data. To store real world data in a lossless state, however, it could require much more. For example, to represent the state of every atom contained in a human body, hundreds of thousands of yottabytes would be required (source).|
Processor Speeds and Frequencies
|Hertz||Hz||Equivalent to one cycle per second.||Screen refresh rates for some types of monitors are measured in Hertz.|
|Kilohertz||KHz||Equivalent to one thousand hertz (one thousand cycles per second).||Audio frequencies are measured in kilohertz.|
|Megahertz||MHz||Equivalent to one million hertz (one million cycles per second).||The processors in the first Apple IIs ran at 1 MHz.|
|Gigahertz||GHz||Equivalent to one billion hertz (one billion cycles per second)||Modern processors are measured in gigahertz. Processors currently available in Apple computers range from 1.6 GHz to 3.2 GHz.|
|Terahertz||THz||Equivalent to one trillion hertz (one trillion cycles per second).||An insanely large frequency. In electronics, it is currently only achieved by experimental transistors.|
FLOPS (Floating Point Operations Per Second) is a measure of performance useful in some areas of scientific calculations. It is not a measure of general computing power, since there are many other factors that influence a computer's speed.
|FLOPS||FLOPS||One floating point operation per second.||Basic handheld calculators could be measured in flops. Each calculation, such as 5.2 * 2.7, is a single floating point operation.|
|KiloFLOPS||KFLOPS||One thousand floating point operations per second.||The term "kiloFLOPS" is not commonly used.|
|MegaFLOPS||MFLOPS||One million floating point operations per second.||Most computers from the 1990's operate in the megaFLOPS, as did the Cray-1 (1976).|
|GigaFLOPS||GFLOPS||One billion floating point operations per second.||Modern personal computers run in excess of one gigaFLOPS.|
|TeraFLOPS||TFLOPS||One trillion floating point operations per second.||Many modern supercomputers, as well as some distributed computing networks (eg Folding@Home) exceed one teraFLOPS.|
Transfer Rates and Bitrates
|Kilobits per second||Kbps||1 kilobit per second is equivalent to 125 (1000/8) bytes per second.||Modems and slower broadband services are measured in Kbps. Music bitrates also use this unit.|
|Kilobytes per second||KBps||1 kilobyte per second is equivalent to 1024 bytes per second.||KBps is not commonly used, but is seen as a more "honest" unit than Kbps since bytes are used more than bits in other places.|
|Megabits per second||Mbps||1 megabit per second is equivalent to 125 KB per second.||This unit is used for faster broadband services, or bitrates for high quality movie files.|
|Megabytes per second||MBps||1 megabyte per second is equivalent to 1024 KB per second.||Extremely fast broadband services will operate at speeds higher than 1 MBps, although the speeds are usually referred to in Mbps.|