File size is a measure of how muchdata acomputer file contains or how muchstorage space it is allocated. Typically, file size is expressed inunits based onbyte. A large value is often expressed with ametric prefix (as inmegabyte andgigabyte) or abinary prefix (as inmebibyte andgibibyte).[1]
Due to typicalfile systemdesign, 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, calledslack space orinternal fragmentation, that is not available for other files but is not used for data in the file to which it belongs.[2]
Generally, a file system allocates space inblocks 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 assize on disk.