article from
http://www.plextor.com/english/support/faqs/G00051.htm

DVD media has the same capacity conversion confusion suffered by users trying to find out the correct capacity of their hard drives. Vendors compute storage size in a decimal value, while computers compute storage in a binary value. One megabyte (MB) of hard drive storage is computed by the vendor as 1 x 106, or 1,000,000. Computers are binary systems, seeing a megabyte as 1 x 220, or 1,048,576 bytes.


In a CD or DVD, a percentage of the disc is taken up by overhead, i;e. link blocks, track information, reserved areas for video title, etc., and takes up some of the user data area , but may not be part of the output data stream. In the case of a DVD, this is approximately 1 MB.


So a DVD disc advertised by the vendor as 4.7GB (decimal) is displayed by computer applications as approximately 4.38GB: (4,700,000,000 ÷ 1,048,576) - 1MB = 4.38GB.