Megabit

v · d · e Multiples of bits
SI decimal prefixes		IEC binary prefixes
Name (Symbol)	Value	Name (Symbol)	Value
kilobit (kbit)	103	kibibit (Kibit)	210 = 1.024 × 103
megabit (Mbit)	106	mebibit (Mibit)	220 ≈ 1.049 × 106
gigabit (Gbit)	109	gibibit (Gibit)	230 ≈ 1.074 × 109
terabit (Tbit)	1012	tebibit (Tibit)	240 ≈ 1.100 × 1012
petabit (Pbit)	1015	pebibit (Pibit)	250 ≈ 1.126 × 1015
exabit (Ebit)	1018	exbibit (Eibit)	260 ≈ 1.153 × 1018
zettabit (Zbit)	1021	zebibit (Zibit)	270 ≈ 1.181 × 1021
yottabit (Ybit)	1024	yobibit (Yibit)	280 ≈ 1.209 × 1024
See also: Nibble · Byte · Multiples of bytes Orders of magnitude of data

The megabit is a multiple of the unit bit for digital information or computer storage. The prefix mega (symbol M) is defined in the International System of Units (SI) as a multiplier of 10⁶ (1 million),^[1] and therefore

1 megabit = 10⁶bits = 1000000bits = 1000 kilobits.

The megabit has the unit symbol Mbit or Mb.

Using the common byte size of 8 bits, 1 Mbit is roughly equal to 125 kilobytes (kB) or approximately 122 kibibytes (KiB).

The megabit is closely related to the mebibit, a unit multiple derived from the binary prefix mebi (symbol Mi) of the same order of magnitude,^[2] which is equal to 2²⁰bits = 1048576bits, or approximately 5% larger than the megabit. Despite the definitions of these new prefixes for binary-based quantities of storage by international standards organizations, memory semiconductor chips are still marketed using the metric prefix names to designate binary multiples.

The megabit is widely used when referring to data transfer rates of computer networks or telecommunications systems. Network transfer rates and download speeds often use the megabit as the amount transferred per time unit, e.g., a 100 Mbit/s (megabit per second) Fast-Ethernet connection, or a 10 Mbit/s Internet access service, whereas the sizes of data units (files) transferred over these networks are often measured in megabytes. To achieve a transfer rate of one megabyte (1000kB) per second one needs a network connection with a transfer rate of eight megabits per second. This can be confusing for internet users assuming the values are in kilobytes and megabytes per second. If one goes to buy a 2 megabit per second internet plan, they are really only getting a 250 kilobyte per second plan.

Usage conventions

• In Telecommunications, use of the correct SI definition of the unit is standard.
• Standard industry practice in RAM and ROM manufacture has been to use the Mb abbreviation in reference to the binary interpretation of the megabit. For example, a single discrete DDR3 chip specified at 512 Mb invariably contains 2²⁹ bits = 536870912bits = 512 Mibit of storage,^[3] or 671088648-bit bytes, variously referred to as either 64 mebibytes or 64 (binary) megabytes.
• During the 16-bit game console era, the megabit was a commonly used measure of the size (computer data storage capacity) of game cartridges. This size represented one mebibit (Mibit). The vast majority of SNES and Mega Drive (Genesis) games were produced on 8 megabit cartridges, although other sizes, such as 4, 12, 16, 24, 32, and 48 megabit cartridges appeared. The standard rule is 8 bits to a 1 byte. Therefore, a 4 megabit cartridge had a capacity of 512 KiB, and a 8 megabit cartridge held 1 MiB of data. Also meaning the 16 bit era was 8 megabits per cartridge.

See also

• Megabyte
• Orders of magnitude (data)
• SI prefix

References

1. ^ The NIST Reference on Constants, Units, and Uncertainty: SI prefixes
2. ^ The NIST Reference on Constants, Units, and Uncertainty: Prefixes for binary multiples
3. ^ "JEDEC Standard DDR3 SDRAM Specification" (PDF, 8.8 MB). http://www.jedec.org/download/search/JESD79-3A.pdf. Retrieved 2008-07-10.