Unformatted and Formatted Capacity

Some portion of the space on a hard disk is taken up by the formatting information that marks the start and end of sectors, ECC, and other "overhead". For this reason, a hard disk's storage total depends on if you are looking at the formatted or unformatted capacity. The difference can be quite significant: 20% or even more.

Older drives that were typically low-level formatted by the user, often had their size listed in terms of unformatted capacity. For example, take the Seagate ST-412, the first drive used on the original IBM PC/XT in the early 1980s. The "12" in this model number refers to the drive's unformatted capacity of 12.76 MB. Formatted, it is actually a 10.65 MB drive.

Now, let's be honest: stating the capacity of the hard disk in unformatted terms is lame. Since nobody can use a drive that is unformatted, the only thing that matters is the formatted capacity. Hard disk manufacturers continue to advertise drives in their unformatted capacities -- for example, a 1TB drive is advertised as a 1TB drive, even though its formatted capacity (NTFS) is only 931GB. BThe manufacturers/vendors have to put a small disclaimer at the bottom of the page in fine print that says "formatted capacity is less" in order to advertise the drives like this. And of course, not everyone reads and/or understands that fine print, so the Internet gets flooded with messages like "why is my 1TB drive only 931GB?!"

Next: Binary vs. Decimal Capacity Measurements