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Size measures

What are kilobytes, megabytes, gigabytes and terabytes? My clients often find these terms confusing.

Bits and Bytes

A bit is an on/off switch, a 1 or a 0. A byte is eight of those. 8 on off bits is enough to offer 256 possibilities.
  • 00 = 0
  • 01 = 1
  • 10 = 2
  • 11 = 3
  • 100 = 4
  • and so on until 11111111 = 255


The prefix kilo means 1,000, which is 10 x 10 x 10. So, a kilobyte (kB) is 1,000 bytes. Notice that kilobyte used this way is abbreviated kB.

However, normally in the computer world, since it is binary, not decimal, kilobyte means 2 raised to the tenth (2^10) which is abbreviated KB and comes to 1024.

  • a Kilobyte (kB) is 1,000 bytes.
  • a Kilobyte (KB) is 1,024 bytes.

My 88,310 byte PDF maintenance form is 86.2 KB while it is 88.3 kB.

Drive and memory makers tend to use the measure that makes their systems seem bigger. Operating systems tend to use the binary measure. That is one reason why a 500 GB drive will show less than 500 GB when Windows reports the size.

Another reason is that drives offer you less space than they have. They don't let you access the entire drive. They reserve space for themselves to replace faulty or failing sections with good ones that are held in reserve. A 500 GB drive they may reserve 25 GB for internal use and spare parts in case of problems. Leaving you with 475 GB of usable space.


A megabyte is 1000 squared 1000^2. While in the computer world, it is 1024^2 or 1,048,576 bytes. For practical purposes, think of it as a million.


A gigabyte is either a billion in the decimal world, 1,000,000,000, but in the binary world it is 1024^3 or 1,073,741,824 bytes.


Terabytes are either a trillion (1000^4 ) or 1024^4 for 1,099,511,627,776 bytes.

 1,000 Bytes 1 Kilobyte
 1,000 Kilobytes
 1 Megabyte
 1,048,576  Millions
 1,000 Megabytes
 1 Gigabyte
 1,073,741,824  Billions
 1,000 Gigabytes
 1 Terabyte
 1,099,511,627,776  Trillions

Putting it in perspective

Bytes in typed pages
 Measure  Typed Pages
 1 kilobyte
 less than half a page
 1 megabyte
 about 419 pages
 1 gigabyte
 about 419,000 pages
 1 terabyte
 about 419,000,000 pages

Date: June 2018

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