It wasn’t very long ago that computer memories used to be calculated in terms of MBs or megabytes, or even KBs or kilobytes! Then we progressed to GBs or Gigabytes of space on our personal computers and then it wasn’t long before you had an 80 GB memory for a device the size of the now all but obsolete audio cassette; a device hardly bigger than the (not so obsolete) cigarette lighter! To add to our incomprehension of these complicated and inscrutable terms, here are some more: Terabyte (TB), Petabyte (PB), Exabyte (EB), Zettabyte (ZB), Yottabyte (YB). So starting from the measly byte we go all the way up to Yottabyte. And just by the way, a byte is 8 bits.

We very glibly use the terms GB or Gigabyte in our everyday conversation, and don’t ordinarily pause to think what exactly the term means. The usage of the word "gigabyte" is not always uniform, it depends on the context within which it is employed. When you talk about RAM size or file size, it usually has a binary meaning, of 1024 raised to the power of three bytes. For other uses, it means precisely 1000 raised to the power of three bytes. That is 1000 multiplied by 1000 multiplied by 1000.

A differentiation has to be made between the following short forms: GB and Gb. While the first refers to Gigabytes, the latter refers to Gigabits.

In current usage there are two main definitions of the term Gigabyte: 1,000,000,000 bytes or 109 bytes is the decimal definition used in telecommunications and/or computer storage makers (hard disks and flash drives). The other definition of the term Gigabyte is
1,073,741,824 bytes, equal to 1024³, or 230 bytes. This is more frequently used for computer memory and file sizes. Manufacturers employ the latter definition to denote the size of their hard drives. This in spite of the fact that the International Electotechnical Commission has recommended the usage of the term gibibyte (shortened to GiB. This recommendation is observed more in the breach however, and the term gigabyte is usually used in everyday speech to denote mean 1024³ bytes. Some people don’t bother to use the recommended terms; the other term is more widely used.

Add to this confusion, there is further perplexity as regards the real or actual capacity of your and my PC hard drive. How is it that my hard drive is actually supposed to be say 10 GB, but when I see the ratio of used to available space in the properties of my computer I see only 9.32 GB of total space. This is because of the discrepancy between definitions as set out above. The true capacity is typically not what the class designation is. Most makers of hard drives describe 1 gigabyte as 1,000,000,000 bytes; the computer operating systems used by most users usually calculate a gigabyte by dividing the bytes by 1,073,741,824. This is the reason that you have at your disposal rather less that the total number of GB that your computer is supposed to have.