File size

File size measures the size of a computer file. Typically it is measured in bytes or bits. The actual amount of disk space consumed by the file depends on the file system. The maximum file size a file system supports depends on the number of bits reserved to store size information and the total size of the file system.

Binary prefixes for powers of 210 (1,024) and decimal prefixs for powers of 103 (1,000) are both in use.[1] The International System of Quantities also allows unambiguous binary prefixes using different names based on an IEC standard, e.g., kibibyte KiB, mebibyte MiB. The traditional KB is usually binary, meaning 1,024 bytes, while the corresponding decimal kB for 1,000 bytes uses a lower case k for kilo.

With typical disk sector sizes of 512, 1,024, 2,048, or 4,096 bytes, decimal prefixes and bits are less suited to describe file systems accurately. For example, the maximal file size on FAT32 is 4GB1B for a binary prefix of bytes, corresponding to 4×1024×1024×10241; decimal 4,294,967,295; hexadecimal FFFFFFFF; or binary 11111111111111111111111111111111 in 32 bits.[2]

Conversion table
Traditional units Decimal for comparison
Name Symbol Binary Number of bytes Equal to Name IEC Decimal Number of bits Equal to
Kilobyte KB 210 1,024 1024 B Kilobit kbit 103 1,000 1000 bit
Megabyte MB 220 1,048,576 1024 KB Megabit Mbit 106 1,000,000 1000 kbit
Gigabyte GB 230 1,073,741,824 1024 MB Gigabit Gbit 109 1,000,000,000 1000 Mbit
Terabyte TB 240 1,099,511,627,776 1024 GB Terabit Tbit 1012 1,000,000,000,000 1000 Gbit
Petabyte PB 250 1,125,899,906,842,624 1024 TB Petabit Pbit 1015 1,000,000,000,000,000 1000 Tbit
Exabyte EB 260 1,152,921,504,606,846,976 1024 PB Exabit Ebit 1018 1,000,000,000,000,000,000 1000 Pbit
Zettabyte ZB 270 1,180,591,620,717,411,303,424 1024 EB Zettabit Zbit 1021 1,000,000,000,000,000,000,000 1000 Ebit
Yottabyte YB 280 1,208,925,819,614,629,174,706,176 1024 ZB Yottabit Ybit 1024 1,000,000,000,000,000,000,000,000 1000 Zbit


  1. JEDEC Solid State Technology Association (December 2002). "Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits" (PDF). JESD 100B.01. p. 8. Retrieved 2009-04-05.
  2. "Microsoft Extensible Firmware Initiative FAT32 File System Specification, FAT: General Overview of On-Disk Format". Microsoft. 2000-12-06. Retrieved 2011-07-03.
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