| Exabits (Ebit) | Kibioctets (Kio) |
|---|---|
| 1 Exabit | 122070312500000 Kio |
| 2 Exabits | 244140625000000 Kio |
| 3 Exabits | 366210937500000 Kio |
| 4 Exabits | 488281250000000 Kio |
| 5 Exabits | 610351562500000 Kio |
| 10 Exabits | 1.220703125 × 1015 Kio |
| 20 Exabits | 2.44140625 × 1015 Kio |
| 25 Exabits | 3.0517578125 × 1015 Kio |
| 50 Exabits | 6.103515625 × 1015 Kio |
| 100 Exabits | 1.220703125 × 1016 Kio |
| Reference | Exabits (Ebit) | Kibioctets (Kio) |
|---|---|---|
| A plain text message (160 characters) | 1.28 × 10-15 Ebit | 0.15625 Kio |
| A three-minute MP3 | 2.4 × 10-11 Ebit | 2929.69 Kio |
| A smartphone photo | 3.2 × 10-11 Ebit | 3906.25 Kio |
| A high-definition film | 0.000000032 Ebit | 3906250 Kio |
| A dual-layer Blu-ray disc | 0.0000004 Ebit | 48828125 Kio |
The exabit is a unit of digital information equal to a thousand petabits, or a billion billion bits. Its symbol is Ebit. At this scale no device, no cable and no building is being described. The exabit measures the traffic of whole regions and the output of entire industries over stretches of time.
An exabit is 125 petaoctets. The most common place to meet the unit is in figures for global internet traffic, which are usually given in exaoctets per month. Worldwide traffic passed one exaoctet per month around 2004, reached about a hundred by 2017 and is now several hundred, which in bits is several thousand exabits every month.
Those totals divide in revealing ways. The overwhelming majority is video, and the rest is dominated by software updates, cloud synchronisation and machine-to-machine traffic rather than by anything a person is actively reading. A single popular film released simultaneously worldwide can move a measurable fraction of an exabit in a day.
Storage at this scale exists only as aggregates. The combined capacity of all the hard drives and flash memory manufactured in a year is measured in hundreds of exaoctets, which is thousands of exabits. Estimates of all the data held by humanity at any moment run into the tens of zettaoctets, and the exabit is the unit one step below that in which the components of such estimates are counted.
Scientific instruments produce exabit quantities before any of it is stored. The Square Kilometre Array radio telescope generates raw data at a rate that would fill exaoctets within days, and its design is built around discarding and summarising in real time rather than recording. The same is true of the large particle detectors and of high-resolution climate simulations.
The prefix exa comes from the Greek for six, the exabit being a thousand to the sixth power in bits. It was adopted in 1975 along with peta, at a time when no one expected either to describe anything but astronomical or theoretical quantities. Both are now ordinary units in the technology industry, which says something about how fast the numbers grew.
One exabit equals 1,000 petabits, 125 petaoctets, or about 0.8674 exbibits.
The kibioctet is a unit of digital information equal to 1,024 octets, and therefore to 8,192 bits. Its symbol is Kio. It is the binary counterpart of the kilooctet, and although the two differ by only 2.4 per cent, this is the unit in which a great deal of a computer's internal organisation is actually measured.
The most important example is the memory page. Processors do not manage memory octet by octet but in fixed-size blocks, and on almost every architecture in common use that block is 4 kibioctets. Every allocation a program makes is rounded up to a multiple of that, every entry in the page tables describes one of them, and the performance of a program often depends on how well its access pattern fits that four-kibioctet grid.
Disc sectors tell a similar story. Hard drives used 512-octet sectors for decades, and modern drives use 4 kibioctets, matching the memory page so that a page can be read or written in a single operation. Filesystems then allocate space in blocks that are themselves powers of two, usually 4 kibioctets, which is why a one-octet file consumes four kibioctets of disc.
Network protocols use the unit too. Buffer sizes, window sizes and the maximum size of many protocol structures are powers of two, and the 64-kibioctet limit appears repeatedly: it is the largest value a 16-bit length field can express, and it therefore caps the size of a UDP datagram, an IP packet and several older file formats.
That is the general pattern: wherever a limit comes from the width of a binary field, the limit is a power of two, and the honest way to write it is with a binary prefix. Writing 64 KB for the datagram limit is not wrong by much, but writing 64 KiB is exactly right and says where the number came from.
In everyday use the difference is invisible. A 200-kilooctet file and a 200-kibioctet file are 4.8 kibioctets apart, which nobody notices. The value of the distinction is that it makes the arithmetic behind a figure legible, which matters far more as the numbers grow.
One kibioctet equals 1,024 octets, 8,192 bits, 8 kibibits, or 1.024 kilooctets.