## Tuesday, 16 July 2013

Somewhere along the way, definitions of hard drive sizes have changed. I'm not sure when and how, but I was brought up to believe that storage capacities were measured in bytes, KB, MB, GB, TB and that these sizes were factors of 2 apart (more specifically, usually 1024x the previous which is 2 ^ 10 or 2 raised to the 10th power).

In short, a byte was 8 bits. A kilobyte (KB) was 1024 bytes, a megabyte (MB) was 1024 KB, a gigabyte (GB) was 1024 MB, and a terabyte (TB) was 1024 GB. Well, apparently this is no longer correct. This is why when I purchase a 750 GB hard drive, it shows up in Windows as having 699 GB! Where did all my storage go!

The answer lies here: Bit and byte prefixes

At some point when I must have been sleeping, the computer industry invented a number of different prefixes which totally confuse what is meant by KB, MB, GB and TB. Apparently, GB refers to GIGAbyte whereas the old definition that I was taught (powers of 2) is now referred to as a GIBIbyte. How odd? Is this because the hard drive manufacturers want to trick people into thinking they are buying a larger drive than they really are? Is every hard drive manufacturer referring to GB as GIGAbyte, or do some use GB to be short for GIBIbyte? Which is it?

So for now:

1000 bytes = 1 kilobyte, and 1024 bytes = 1 kibibyte.
1000 kilobytes = 1 megabyte, and 1024 kibibytes =  1 mebibyte
1000 megabytes = 1 gigabyte, and 1024 mebibytes =  1 gibibyte
1000 gigabytes = 1 terabyte, and 1024 gibibytes =  1 tebibyte

So by the time you get up to 750 GB, depending on what you are defining as "GB" (gigabyte or gibibyte) you end up with the following numbers of bytes:

750 x 1000 x 1000 x 1000 = 750000000000 bytes (750 gigabytes)
750 x 1204 x 1024 x 1024 = 805306368000 bytes (750 gibibytes)

Windows obviously handles things the traditional way, which is why it is reporting my 750 GB as only 699 GB. If you take 750,000,000,000 divided by 1024x1024x1024 you end up with 698.5 GB, which is what Windows is reporting, essentially. So what happened?

Windows is obviously using the traditional definition with factors of 2 to some power (1024 x) whereas hard drive manufacturers must be trying to mislead the public by over-inflating their hard drive sizes by using a power of 10 (1000 x). How do you know which company is using which definition, when the box only states GB or TB and doesn't state whether that is the new or old definition?