Computing units

Use the same rules – for abbreviations, and for words versus numerals – as for scientific units.

12 MB     16 GB     23 Kibits/s

[Computer memory is usually expressed in gigabytes.]

This section covers:

Data

The basic unit of data is the binary bit (symbol: bit), which can take on a value of 1 or 0. A byte (B) consists of 8 bits.

According to the International System of Quantities, a kilobyte is 1,000 bytes. The recommended symbol is kB. This usage agrees with the International System of Units, which is the modern form of metric measurement. The same is true of mega- (M), giga- (G) and other prefixes.

Because binary arithmetic is so important in computing, kilobytes were traditionally (and continue to be) taken by some companies and users as 210 = 1,024 bytes. This is now more properly referred to as a kibibyte (KiB – note the capital K), where the bi denotes the binary origin of the value. Similarly, we have mebibytes (MiB), gibibytes (GiB) and so on.

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Data rates

When transferring data (eg over the internet), speeds are typically expressed in bits per second. The unit should in general be written bit/s. The form bps is widely used in material for a general readership, where its similarity to kph (kilometres per hour) may be an aid to understanding. In more formal writing, bits/s is preferred, because it uses the correct unit symbol (bit) and is more in keeping with the standard way of writing units of measure:

The NBN typically delivers data rates above 40 Mbps.
or (more correctly)
The NBN typically delivers data rates above 40 Mbit/s.

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Common units

Unit

Symbol

Definition

bit

bit

A single binary data element; 1 or 0

byte

B

8 bits

kilobyte

kB

1,000 B

megabyte

MB

1,000 kB

gigabyte

GB

1,000 MB

terabyte

TB

1,000 GB

petabyte

PB

1,000 TB

kilobit

kbit

1,000 bits

kilobits per second

kbit/s

1,000 bits per second

megabits per second

Mbit/s

1,000 kbits per second

kibibyte

KiB

1,024 B

mebibyte

MiB

1,024 KiB

gibibyte

GiB

1,024 MiB

tebibyte

TiB

1,024 GiB

pebibyte

PiB

1,024 TiB

kibibit

Kibit

1,024 bits

kibibits per second

Kibit/s

1,024 bits per second

mebibits per second

Mibit/s

1,024 Kibit per second
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Frequency

Computers are coordinated by an internal clock. The computer state is updated each time the clock ticks. So, the faster the clock, the faster the computer.

Frequencies are expressed in cycles per second, or hertz (Hz). Hertz is a measure derived from the International System of Units.

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