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Whether the prefixes kilo-, mega-, giga-, etc., and their abbreviations K, M, G, etc., are being used in the binary or the decimal sense depends on context and common usage. This can cause serious confusion.
| Name | Symbol | Value | Base 16 | Corresponding SI prefix value |
|---|---|---|---|---|
| 210 = 1 024 | = 162.5 | 103 = 1 000 | ||
| 220 = 1 048 576 | = 165 | 106 = 1 000 000 | ||
| 230 = 1 073 741 824 | = 167.5 | 109 = 1 000 000 000 | ||
| 240 = 1 099 511 627 776 | = 1610 | 1012 = 1 000 000 000 000 | ||
| 250 = 1 125 899 906 842 624 | = 1612.5 | 1015 = 1 000 000 000 000 000 | ||
| 260 = 1 152 921 504 606 846 976 | = 1615 | 1018 = 1 000 000 000 000 000 000 | ||
| 270 = 1 180 591 620 717 411 303 424 | = 1617.5 | 1021 = 1 000 000 000 000 000 000 000 | ||
| 280 = 1 208 925 819 614 629 174 706 176 | = 1620 | 1024 = 1 000 000 000 000 000 000 000 000 |
The one-letter abbreviations are identical to SI prefixes, except for "K", which is used interchangeably with "k". (In SI, "K" stands for the Kelvin unit, and only "k" stands for 1000.) Some have suggested that "k" be used for 1000, and "K" for 1024, but this is not extended to the higher order prefixes and has never been widely recognised.
Notice that as the order of magnitude increases, the percentage difference between the binary and decimal values of a prefix increases.
Informally, the prefixes are often used on their own. Thus one might hear about "a 40K file" (40 binary kilobytes) or "a 2M internet connection" (2 decimal megabits per second). What units are being used, and whether the multipliers are decimal or binary, depends on exactly what is being measured.
Certain units always use the decimal versions, even in computing contexts. Most importantly, the Hz, used to measure clock rates of electronic components, and the bit/s, used to measure bitrate. So a 1 GHz processor performs 1,000,000,000 clock ticks per second, a 128 kbit/s MP3 stream consumes 128,000 bits (15.625 KiB) per second, and a 1 Mbit/s internet connection can transfer 1,000,000 bits (approx. 122 KiB) per second.
Electronic memory such as RAM and ROM always uses the binary versions, because the physical structure of the device makes it naturally come in sizes that are powers of two. This is the case whether the capacity is given in bits or bytes.
Hard disk drives use the decimal prefixes. So what is advertised as a "30 GB" hard disk will actually only hold 30 × 109 bytes, roughly equal to 28×230 bytes (i.e. 28 GiB). This makes sense because nothing about the physical structure of the disk drives makes power-of-two capacities natural: the number of platters, tracks and sectors per track are all continuously variable. Modern-day PC users, of course, regard both RAM and disk as kinds of storage and expect their capacities to be measured in the same way.
Floppy diskA floppy disk is a data storage device that comprises a circular piece of thin, flexible (hence "floppy") magnetic storage medium encased in a square or rectangular plastic wallet. Floppy disks are read and written by a floppy disk drive or FDD not to bes are even more confusing. The "1.44 MB" floppy holds neither 1.44 × 220 bytes nor 1.44 × 106 bytes, but rather 1.44 × 1000 × 1024 bytes (approximately 1.406 binary MB , or 1.475 decimal MB).
CDCD re-directs here; see Cd for other meanings of CD . A compact disc (or CD is an optical disc used for storing digital data. It was originally invented for digital audio and is also used as a data storage device, a CD-ROM. CD-ROM reading devices are a st capacities are always given in binary units. A "700 MB" (or "80 minute") CD has a nominal capacity of about 700 MiB. But DVDDVD is an optical disc storage media format that is used for playback of movies with high video and sound quality and for storing data. DVDs are similar in appearance to compact discs. History During the early 1990s there were two high density optical sto capacities are given in decimal units. A "4.7 GB" DVD has a nominal capacity of about 4.38 GiB.
Note that all the above media are accessed by the sector, not the individual byte. Sectors are always powers of two, and may range from 512 bytes (floppy disks) to 2048 bytes (DVDs). This has sometimes led to mixed conventions within certain specialties, in which a nominal "megabyte" refers to, e.g. 2000 sectors of 512 bytes each (210 × 103).
Decimal versions are used when dealing with busIn computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. bandwidth (e.g. "Ultra SCSI has a bandwidth of 40 megabytes per second"). Interestingly, this is not because hard drive capacities use the decimal versions, or because bitrates do, but because clock speeds do.