• File size is a measure of how much data a computer file contains or, alternately, how much storage it consumes. 
  • Typically, file size is expressed in units of measurement based on the byte. 
  • By convention, file size units use either a metric prefix (as in megabyte and gigabyte) or a binary prefix (as in mebibyte and gibibyte).
  • The smallest unit in computers is bit and comes from binary digit.
  • The bits are combined in groups in order to form larger units. The next unit larger than the bit is the byte which is formed by the combination of eight bits and can represent a value from 0 to 255 which is 2 to the power of 8 - all the possible combinations of the 8 bits that it includes.
  • The next larger units after the byte are named kilobyte, megabyte, gigabyte, terrabyte and so on which lead to great deal of confusion. 
  • Though the kilo prefix in the metric system means 1000 in computers it means 1024.
  • Here is a list of some of the commonly used units in the metric and their corresponding binary prefixes:
  • Table of approximate file sizes:
  • In the picture below of files listed in Windows Explorer, the size column has been sorted to show the largest file first.
  • As can be seen, the first file "family-safety.jpg" has a file size of 47 KB. 
  • The smallest file in this picture has a file size of 32 KB.
  • File transfers may use rates of units of bytes (e.g. MB/s) in binary rather than metric system.
  • While networking hardware, such as WiFi, always uses the metric system.