| Megabits (Mbit) | Pebioctets (Pio) |
|---|---|
| 1 Megabit | 1.11022302463 × 10-10 Pio |
| 2 Megabits | 2.22044604925 × 10-10 Pio |
| 3 Megabits | 3.33066907388 × 10-10 Pio |
| 4 Megabits | 4.4408920985 × 10-10 Pio |
| 5 Megabits | 5.55111512313 × 10-10 Pio |
| 10 Megabits | 0.00000000111022302463 Pio |
| 20 Megabits | 0.00000000222044604925 Pio |
| 25 Megabits | 0.00000000277555756156 Pio |
| 50 Megabits | 0.00000000555111512313 Pio |
| 100 Megabits | 0.0000000111022302463 Pio |
| Reference | Megabits (Mbit) | Pebioctets (Pio) |
|---|---|---|
| A plain text message (160 characters) | 0.00128 Mbit | 1.42109 × 10-13 Pio |
| A three-minute MP3 | 24 Mbit | 0.00000000266454 Pio |
| A smartphone photo | 32 Mbit | 0.00000000355271 Pio |
| A high-definition film | 32000 Mbit | 0.00000355271 Pio |
| A dual-layer Blu-ray disc | 400000 Mbit | 0.0000444089 Pio |
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 pebioctet is a unit of digital information equal to 1,125,899,906,842,624 octets, which is 1,024 tebioctets. Its symbol is Pio. It is the binary counterpart of the petaoctet, and the two now differ by 12.6 per cent.
This is the unit in which filesystem limits are written. Several widely deployed filesystems have maximum file or volume sizes that are exact powers of two, and those limits fall in this range: sixteen tebioctets for one, a pebioctet for another, eight pebioctets for a third. The numbers are not choices made by anyone; they follow from how many bits the filesystem uses to number its blocks.
Large scientific and commercial storage systems are also sized here. A research computing centre may run a filesystem of several pebioctets across thousands of drives, and the tools that manage it report in pebioctets because the underlying allocation is binary throughout. Converting those figures to decimal units for a report introduces an error of an eighth for no benefit.
A pebioctet is roughly a thousand large consumer hard drives, or the complete video output of a broadcaster over many years. It is also close to the size of the largest single scientific archives, though those are typically stated in petaoctets by press offices and in pebioctets by the engineers who run them, which produces two different numbers for the same building.
Distinguishing the two is not pedantry at this level. A twelve per cent difference on a purchase of many pebioctets is a large sum of money, and a capacity plan built on the wrong unit will be wrong by more than the margin most plans allow. This is precisely the situation the IEC prefixes were introduced to prevent.
The symbol Pio is used in filesystem documentation, in storage system manuals and in standards. Where a source writes PB without saying which it means, the difference is an eighth, and the honest thing for a reader to do is treat the figure as approximate rather than assume one convention or the other.
One pebioctet equals 1,024 tebioctets, 1,125,899,906,842,624 octets, 8 pebibits, or about 1.126 petaoctets.