| Megabits (Mbit) | Exbioctets (Eio) |
|---|---|
| 1 Megabit | 1.08420217249 × 10-13 Eio |
| 2 Megabits | 2.16840434497 × 10-13 Eio |
| 3 Megabits | 3.25260651746 × 10-13 Eio |
| 4 Megabits | 4.33680868994 × 10-13 Eio |
| 5 Megabits | 5.42101086243 × 10-13 Eio |
| 10 Megabits | 1.08420217249 × 10-12 Eio |
| 20 Megabits | 2.16840434497 × 10-12 Eio |
| 25 Megabits | 2.71050543121 × 10-12 Eio |
| 50 Megabits | 5.42101086243 × 10-12 Eio |
| 100 Megabits | 1.08420217249 × 10-11 Eio |
| Reference | Megabits (Mbit) | Exbioctets (Eio) |
|---|---|---|
| A plain text message (160 characters) | 0.00128 Mbit | 1.38778 × 10-16 Eio |
| A three-minute MP3 | 24 Mbit | 2.60209 × 10-12 Eio |
| A smartphone photo | 32 Mbit | 3.46945 × 10-12 Eio |
| A high-definition film | 32000 Mbit | 0.00000000346945 Eio |
| A dual-layer Blu-ray disc | 400000 Mbit | 0.0000000433681 Eio |
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 exbioctet is a unit of digital information equal to two to the sixtieth power octets, which is 1,024 pebioctets. Its symbol is Eio. It is the binary counterpart of the exaoctet, and the two differ by 15.3 per cent.
One number involving this unit is quoted more than any other: sixteen exbioctets, the size of the address space a 64-bit processor can reach. Two to the sixty-fourth octets is 16 EiB, and that figure is the theoretical ceiling on the memory of every machine built on the architecture that has dominated computing since the mid-2000s.
The same limit reappears in filesystems. A filesystem that numbers its blocks with 64-bit values can address 16 exbioctets of blocks, and several modern designs state exactly that as their maximum volume size. Others state 8 exbioctets, having reserved one bit for a sign or a flag — a detail that halves the limit and is worth knowing when reading a specification.
Nothing approaches these sizes in practice. The largest storage systems in the world hold exaoctets, and the total of all data held by humanity is a few hundred zettaoctets, which is tens of thousands of exbioctets. The 64-bit ceiling was chosen precisely so that it would not be reached, and the transition from 32 bits, whose four-gibioctet limit was reached within a decade, is why the designers left so much room.
An exbioctet is 1,152,921,504,606,846,976 bits and 128 pebioctets. It is a quantity that exists in specifications, in address arithmetic and in the design documents of large systems, and nowhere else. Describing it in terms of photographs or films is not useful, because no collection of either comes close.
The symbol Eio and its decimal sibling Eo differ by more than an eighth, which is far too much to leave to inference. When a document writes EB in the context of an address space, it almost certainly means EiB, because address spaces are powers of two by construction; when it writes EB about stored data, it almost certainly means the decimal unit.
One exbioctet equals 1,024 pebioctets, 1,152,921,504,606,846,976 octets, 8 exbibits, or about 1.153 exaoctets.