| Megabits (Mbit) | Zebibits (Zibit) |
|---|---|
| 1 Megabit | 8.47032947254 × 10-16 Zibit |
| 2 Megabits | 1.69406589451 × 10-15 Zibit |
| 3 Megabits | 2.54109884176 × 10-15 Zibit |
| 4 Megabits | 3.38813178902 × 10-15 Zibit |
| 5 Megabits | 4.23516473627 × 10-15 Zibit |
| 10 Megabits | 8.47032947254 × 10-15 Zibit |
| 20 Megabits | 1.69406589451 × 10-14 Zibit |
| 25 Megabits | 2.11758236814 × 10-14 Zibit |
| 50 Megabits | 4.23516473627 × 10-14 Zibit |
| 100 Megabits | 8.47032947254 × 10-14 Zibit |
| Reference | Megabits (Mbit) | Zebibits (Zibit) |
|---|---|---|
| A plain text message (160 characters) | 0.00128 Mbit | 1.0842 × 10-18 Zibit |
| A three-minute MP3 | 24 Mbit | 2.03288 × 10-14 Zibit |
| A smartphone photo | 32 Mbit | 2.71051 × 10-14 Zibit |
| A high-definition film | 32000 Mbit | 2.71051 × 10-11 Zibit |
| A dual-layer Blu-ray disc | 400000 Mbit | 3.38813 × 10-10 Zibit |
The megabit is a unit of digital information equal to one million bits. Its symbol is Mbit. It is the unit in which the speed of an internet connection is almost always advertised, which makes it one of the few data units that ordinary consumers encounter by name every time they choose a service.
A megabit is one million bits exactly, not 1,048,576. That distinction has practical consequences. A megabit holds 125,000 octets, which is 125 kilooctets, so a connection running at 100 megabits per second transfers about 12.5 megaoctets per second at best. A file listed as 500 megaoctets therefore takes a minimum of forty seconds, not the four the advertised number seems to promise.
That factor of eight is the reason so many people believe their connection is slower than they were sold. Nothing dishonest is happening: the industry quotes throughput in bits per second because that is what the physical layer actually carries, while file managers quote size in octets because that is how storage is organised. Both conventions are correct in their own domain, and the arithmetic between them is a division by eight.
Real throughput is lower still. Protocol headers, error correction and retransmission all consume capacity, and the usable share of a link is typically 90 to 95 per cent of its nominal rate. Wireless links lose more, because the medium is shared and interference forces retries. A connection advertised at 100 megabits per second commonly delivers 90 or so in practice, and less over a busy wireless network.
The numbers that define the eras are worth remembering. Early broadband offered 1 to 8 megabits per second, cable and fibre pushed that to 50 and 100, and gigabit services are now common in cities. High-definition video streaming needs roughly 5 megabits per second, ultra-high-definition roughly 25, and a video call between two 8, so a household's real requirement is usually far below what it buys.
In memory the megabit describes chip capacity. A 512-megabit memory chip holds 64 megaoctets, and several such chips make a module. Manufacturers count in bits because that is what the silicon holds; buyers count in octets because that is what the operating system reports.
One megabit equals 1,000,000 bits, 1,000 kilobits, 125 kilooctets, or about 0.9537 mebibits.
The zebibit is a unit of digital information equal to two to the seventieth power bits, which is 1,024 exbibits. Its symbol is Zibit. It is the binary counterpart of the zettabit, and the two now differ by 18.1 per cent — nearly a fifth.
That divergence is the reason the binary prefixes were needed at all. At the kibibit the two conventions differed by 2.4 per cent, a rounding error. Here the difference is large enough that a document using the wrong one is simply reporting a different quantity, and no amount of context can repair the ambiguity after the fact.
A zebibit is 147,573,952,589,676,412,928 octets, or 128 exbioctets. Nothing of this size exists. The total data held by humanity is somewhere in the low hundreds of zettaoctets, which is under a zebioctet, so the world's entire information stock does not yet reach one unit at this step of the binary ladder.
The unit exists because the IEC series was defined completely rather than as far as anyone then needed. That is the same principle the metric system follows: every prefix applies to every unit, whether or not the combination has yet been used. A system with holes in it requires a table of exceptions, and a system without holes requires only the rule.
Where the zebibit could genuinely appear is in address space arithmetic. Two to the seventieth is not a natural pointer width, but multiples and fractions of powers of two run through every discussion of addressing, and a scheme that reserved seventy bits for something would naturally be described in these terms. Such schemes are proposed occasionally and none has yet been needed.
Reading the symbol is the practical skill. Zibit is binary, Zbit is decimal, and at eighteen per cent apart the two are not interchangeable in any document where the number matters. Where a source writes ZB with no explanation, there is no way to know which was meant, and the honest response is to treat the figure as uncertain to a fifth.
One zebibit equals 1,024 exbibits, 147,573,952,589,676,412,928 octets, or about 1.181 zettabits.