When you boot up Snow Leopard, you might be surprised to find your total disk space bigger than it was before. Your 320 GB hard-drive might have suddenly jumped from a mere 299 to a near full 319. What happened?

No, your drive space didn’t magically get bigger because of an magic trick by Apple, rather, the way the disc size is reported has been changed to fit new data representations standards.

Traditionally, like most companies, Apple used to report the data the wrong way, or the old way, depending on who you talk to. The wrong/old way says that any multiple of the “byte” measure x1000 and up is actually 1024 times its value. For example, 1 KB (Kilobyte) would equal 1024 B (Bytes). However, in the new standards, given confusion with SI units which normally represent 1000 of a given item and not 1024, 1 KB has been rectified to be 1000 B. To illustrate the other measure still widely used in the industry for reasons beyond the scope of this article, a new measure was invented. In the case of the KB, we call this one Kibibyte (KiB).

Some Linux distributions chose to change their measure from KB to KiB, MB to MiB, GB to GiB, etc. and retain the same numbers. Others, like Apple, have chosen to avoid potential user confusion with the added small “i” and change the numbers instead to correctly represent a GB for example, as 1000 MB, and so on. Other manufacturers like Microsoft have yet to change and still use the wrong measures.

What happens to the hard drive is that advertised disc sizes are always in standard SI format GB. 320 GB is thus really 320,000,000,000 B. However, the way your OS, such as Windows, reads it, is typically the wrong way, GB (Giga) as GiB (Gibi), meaning 320,000,000,000 B reduced gives 298 GB, which is in fact 298 GiB. If you ever wondered why the advertised size of your disk was always smaller in real life, that’s the real reason.

Chapeau, Apple, for making it right.