Have I lost some space on my disk?

I bought some blank DVDs to store backup files. The disks say that they hold 4.7Gb of data, but when I put one in my computer, Vista shows it having 4.3Gb of space. What happened to the other 0.4Gb?

Underside of a DVD-R disc

The discrepancy is caused by the fact that there are 2 ways of defining computer memory. Let’s start from the beginning, technically speaking, this is how computer memory is measured:

  • 1 Kilobyte is 1,024 bytes (2 to the power of 10)
  • 1 Megabyte is 1,048,576 bytes (2 to the power of 20)
  • 1 Gigabyte is 1,073,741,824 bytes (2 to the power of 30)

Now, as you can imagine, those number are pretty hard to remember, so most people think of 1 Kilobyte as 1,000 bytes, 1 Megabyte as 1,000,000 bytes and 1 Gigabyte as 1,000,000,000 bytes. The disk manufacturers are using the former system, which is technically more accurate but also has the side effects of making their disks look “bigger”. Vista, on the other hand uses the latter system, to make things easier for the user. So, you haven’t really lost any space, it’s just a different way of counting the same thing.

1 thought on “Have I lost some space on my disk?”

  1. Is it possible you’ve got those the wrong way round, as in manufactures use 1,000 = 1K, as opposed 2^10=1K?

    Also, I recently discovered that despite my moans in the past, if you wanted to be a real pedant then the disc/disk manufacturers are correct – 1024 bytes are apparently 1 kibibyte, 1024 kibibyte are a mebibyte and so on. http://www.iec.ch/zone/si/si_bytes.htm explains it far better than I can.

    Mind you, its a bit like the hacker vs. cracker terminology debate – it doesn’t matter how many people you tell that hackers are good people who write code for companies, and crackers are often evil, a tiny proportion of people use the phrases in that way…

    Also, don’t forget that actually formatting a disk, be it floppy/optical or hard drive, will always ‘lose’ some more capacity, as the overheads from FAT/NTFS/etc. etc. (and other necessary parts like MBR in some cases) have to be taken into account.

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