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The mystery of the hard drive capacity. Or the proper use of standard office

In recent years, computer users are surprised by the difference between the stated capacity by hard drive manufacturers, and actual capacity announced by the operating system. Thus, a 80GB drive only shows he that 74.5 GB in Windows.

Whence the difference? Some explanations heard here and there talk about the effect of formatting, which would "lose" space on the disk. Why not? But loss of nearly 7% for formatting, that's a lot.

And if the truth was both simpler and more obscure? Simple as mathematics, and dark as a truth ignored by the media.

blame the IEC!

Everything is explained on the website of the IEC , venerable International Electrotechnical Commission (or International Electrotechnical Commission).

The IEC is the source of a standard, in force since December 1998, which adds to the Système International (SI) a new standard "binary prefixes".

recall here that the SI defines a set of seven basic units: the meter for length, for the second time, the ampere for electric current, kelvin for temperature, the mole quantities of matter and the candela for luminous intensity.

It also defines the SI multiples and their abbreviations, yotta (10 power 24) to yocto (10 power -24) including of course per kilo, mega, giga.

kilo, mega, giga

Precisely, these prefixes are well known to computer science have long been used (incorrectly) to mean multiples of 1024 (2 ^ 10) instead of 1000 (10 ^ 3). In the early days of computing, the difference was not great, but with higher capacity, it becomes more visible. 2.4% across the kilo it is 20% across the exam.

Since 1998, therefore, the IS clearly defines the kilobyte has 1000 bytes and not 1024. To please the computer, the IEC had the good taste to create new prefixes for binary multiples. We must therefore use the kilobinaire to designate 1024, which gives the kibibyte (KB) for 1024 bytes. Binary multiples are as follows (click on picture to enlarge):



So the explanation of bytes lost in "format": in fact, drive manufacturers offer their capacity in kilobytes, while the operating system himself, account kibibyte without saying. When Windows says the drive has 74.5 GB, it should be understood Gio 74.5, 74.5 * 2 ^ 30 (or 79.99 GB).

It is therefore urgent that everyone get into the habit of speaking the same language, or at least the same measures. Either we accept that a kilogram is 1000 bytes as in all other measures, or we must learn to handle binary multiples.

I confess to a weakness for the latter option. Just to poetry: what a pleasure to handle the strips of 256 mebioctets, tebibytes several disks, or network flow more gibibits! The Shadocks would have loved ...