Advertised disk space vs actual disk space [closed]

Question

Why is it that advertised disk space is almost always higher than the disk space reported by the UI? For example, I have an "80 gb" hard drive, but the iTunes UI indicates only 74. I usually see this as well with hard disks and the amount reported with the drive letter.

Pilliwinks

Great question, somebody should migrate/copy this over to SuperUser. – Peddle

A new Western Digital Passport drive I just got is both marketed and showing as 1TB. Has there been a glitch in the matrix? Has this practice finally ended? – Urina

@JohnHimmelman So vote to close, mark as off topic and it will be moved. (If you are still here after 2.5 years...) – Estrone

@Aphex5 Probably the "showing as 1TB" now works correct and doesn't erroneously display TiB... – Estrone

@Estrone Thanks, just voted. I didn't have mod privileges 2.5 years ago ;) – Peddle

@Estrone You're right, turns out Macs calculate storage with gigabytes / terabytes, not gibibytes / tebibytes. Actually I've been asleep at the wheel - this change happened in (gulp) 2009, with Snow Leopard. – Urina

There is a little function: where l is first letter after digits: i=index(l,"bKMGTPEZY")-1; binary=vendor*(10^(3*i)/2^(10*i)). For 80Gb, i=3, then bc -l <<<'80*(10^(3*3))/2^(3*10)' give: 74.5058.. . I wrote a french javascript there for helping computing sizes. – Garvin

Answer 1

There are 3 reasons why the amount of space you can actually use is different from that listed for the drive, all of which work against you:

1. Hard drive manufacturers treat 1GB as one billion bytes, while the operating system calls it 1,073,741,824 bytes (1000 * 1000 * 1000 vs 1024 * 1024 * 1024).
2. You lose some space for file tables when formatting.
3. Disk space is divided into chunks (blocks) larger than 1 byte (typically 4K). Using typical Windows defaults, even just a 1 byte file still uses 4K of space on disk. A 4097 byte file uses 8K.

Of these, the first two can influence the amount of space reported by the drive (though IIRC the 2nd one was more of an issue with FAT32 than NTFS). The last one only influences the amount of free space remaining, but will still prevent you from using the full capacity of your drive.

Prato

I'm not sure that this is true anymore. There have been some class action lawsuits over this marketing gimmick. – Catalysis

Actually, this is true. And technically, the HD manufacturers are right -- a gigabyte is 1 billion bytes, a gibibyte is 1,073,741,824 bytes. The OS reports gibibytes, the HD manufacturers gigabytes, and never the twain shall meet. See Wiki: en.wikipedia.org/wiki/GiB – Demitasse

(Continued from above ... ) Also see Wiki on Gigabytes, which details the controversy: en.wikipedia.org/wiki/Gigabyte – Demitasse

+ The partition windows is installed on doesn't get counted. – Cacology

Answer 2

It's the way the OS calculates free space vs the hard drive manufacturers.

OS: 1mb = 1024 kb

Vendor: 1mb = 1000 kb

The vendor will always use the *1000 to increase their numbers.

Joanejoanie

That's why kiB, MiB, GiB, TiB, ... was invented back in 1998... – Estrone

Answer 3

The main culprit is using base 10 vs. base 2 to list the storage size. It effectively becomes a rounding error.

There is a movement to try and list storage size with base 2 values instead of base 10 to reflect the true size.

Lugar

Answer 4

It's the difference between the standard (SI) prefixes (giga, mega, kilo, etc.) which are multiples of 1000 and the binary prefixes which are multiples of 1024.

Fadden

Answer 5

Marketing considers 80 gigabytes to be 80,000,000,000 bytes. The OS considers 80 gigabytes to be 85,899,345,920 bytes.

http://www.google.com/search?q=80000000000+bytes+in+GB

Avalon

Answer 6

Usually due to some partitioned space that the OS or some software takes and hides for backup or system purposes.

Rennes

Good answer to the wrong question. That wouldn't affect the advertised size. It simply limits the available space when installed. – Lugar

I think this is a fair possibility to consider of why advertised space can be higher than actual realized space. – Guyguyana

Answer 7

Say manufacturer consider a MB to be 1024KB; others 1000KB. Similarly for GB. Some say 1024MB; others 1000MB.

Then, that refers to the un-formatted size. Formatting takes up some space.

Inness

Answer 8

Additionally many times they advertise gigabytes as slightly inaccurate numbers, which result in differences. You can see this in the disclaimer text on the outside of most hard drive boxes!

Australorp