Advertised disk space vs actual disk space

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.

Answer 1

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

Answer 2

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.

Answer 3

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!

Answer 4

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.

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

Answer 6

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 manufactures 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 larger than 1 byte (typically 4K). Using typical Windows defaults, a 1 byte file takes up 4K of space on disk.

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 80GB drive.

Answer 7

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.

Answer 8

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.