Byte

Also called: octet, 8 bits

A group of 8 bits, the standard unit of memory and file size. A byte holds 256 patterns: 0 to 255 unsigned, or −128 to 127 in two's complement.

A byte is 8 bits. It's the unit computers use to measure memory, files and data, and each memory address usually holds one byte (see byte addressing).

What fits in a byte:

2⁸ = 256 different patterns
unsigned: 0 to 255
twos complement: −128 to +127
exactly two hexadecimal digits, 00₁₆ to FF₁₆, because each hex digit is one 4-bit nibble

That last point is why bytes are almost always written in hex. A web color like #1E90FF is three bytes (red, green, blue), and a debugger shows memory as pairs of hex digits.

One character of plain English text is traditionally stored in one byte. In the ASCII code, the letter A is 65 = 01000001.

On units: in memory sizes, a kilobyte is commonly 2¹⁰ = 1024 bytes. Network speeds count bits, not bytes, so watch the lowercase b in "Mb/s".

128

Worked example

Example

One byte read three ways: 11001000

The same 8 bits, under three different readings.

1.
Unsigned: 128 + 64 + 8 = 200.
2.
Hex: split into nibbles, 1100 | 1000, giving C8₁₆.
3.
Two's complement: the MSB is 1, so the value is 200 − 256 = −56.
4.
The bits don't change. The format you choose decides the value.

Common mistakes

Saying a byte holds values up to 256. The largest unsigned byte is 255.
Mixing up bits and bytes: 8 megabits is 1 megabyte.
Writing a byte as a single hex digit. A byte always needs two, one per nibble.

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Byte is taught in Number Systems and Memory.

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