File size is a measure of how much
data
Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...
a
computer file
A computer file is a System resource, resource for recording Data (computing), data on a Computer data storage, computer storage device, primarily identified by its filename. Just as words can be written on paper, so too can data be written to a ...
contains or how much
storage space it is allocated. Typically, file size is expressed in
units based on
byte. A large value is often expressed with a
metric prefix
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
(as in
megabyte and
gigabyte
The gigabyte () is a multiple of the unit byte for digital information. The SI prefix, prefix ''giga-, giga'' means 109 in the International System of Units (SI). Therefore, one gigabyte is one billion bytes. The unit symbol for the gigabyte i ...
) or a
binary prefix (as in
mebibyte
The byte is a units of information, unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character (computing), character of text in a computer and for this ...
and
gibibyte).
Slack space
Due to typical
file system design
A design is the concept or proposal for an object, process, or system. The word ''design'' refers to something that is or has been intentionally created by a thinking agent, and is sometimes used to refer to the inherent nature of something ...
, the amount of space allocated for a file is usually larger than the size of the file's data resulting in a relatively small amount of storage space for each file, called slack space or internal fragmentation, that is not available for other files but is not used for data in the file to which it belongs.
Generally, a file system allocates space in
blocks that are significantly larger than one byte. The file system allocates a number of blocks that together provide enough space to hold the file data. Unless the file fits exactly into the aggregated blocks, then some storage space allocated to the file is unused by the file.
A file's allocated storage size is sometimes referred to as file size or alternatively with qualification such as ''size on disk''.
See also
*
References
{{Computer files
Computer files