Q Why is it that when I put a blank 4.7GB DVD-RW into my notebook’s DVD burner, Windows Vista only recognises the blank disc as having a capacity of 4.3GB. This is before the disc has been formatted. Why is there a reduction of 0.4GB in usable disc space before anything has been recorded?
Bill Harper-Wright

A There are several reasons for this. The first is that the 4.7GB quoted is the absolute maximum amount of space available for storage on the disc. When the disc is formatted by the computer, it uses a certain amount of space to set up the various structures that are required in order to be able to store files. This accounts for some of the missing space.

Most of the difference is attributable to the different ways of counting computer storage space. To cut a long story short, it’s possible to count a gigabyte as either 1,000,000,000 bytes or 1,073,741,824 bytes. So 4.7GB is 4.7 multiplied by 1,000,000,000, but if you divide 4,700,000,000 by the second number above you’ll get the 4.3 figure as the computer reports it.

So the industry is able to define a DVD as having either 4.7GB or 4.3GB, depending on which measure you choose to use, and of course the industry chooses the one that makes the capacity of the disc sound bigger. This is a common source of confusion among consumers and the industry alike, but the short answer to your question is that the 0.4GB of missing space isn’t missing because it was never there to begin with.